Integrated & power amplifier matching

J

Johnny Rega

Audiophyte
I have just purchased a new Arcam P49 power amplifier from a UK retailer. As it was a store demo and I was able to deduct VAT, I got a fantastic deal. I've connected this to my Rega Elicit-R amplifier, which I'm using as a pre-amp, to drive a pair of KEF Q900s. The combination sounds really amazing. But when the Rega is set to 12 o'clock or higher, the P49 shuts off automatically.

Could somebody please explain to me why this is happening? Is there some type of gain mismatch going on? The retailer is now suggesting I buy a step down attenuator. Or did the retailer just send me a dud P49?

Thanks for helping me understand what's going on.

-Johnny
 
P

PENG

Audioholic Slumlord
Did you set the gain switch to 25 or 31 dB? It also has an input XLR/RCA selector switch that should be set to RCA because the Rega has RCA pre-outs only.

The important is, how loud was it just before it shut down? If you have a spl meter, find out how many dB you are getting with volume at 12, at distance of 1 meter from your speaker (L or R). Better yet if you can measure the voltage at the speaker's terminal.

If I were to guess, you were probably just over driving the power amp, causing it to shutdown on thermal protection.

The retailer's suggestion of a step down attenuator makes no sense at all.
 
highfigh

highfigh

Seriously, I have no life.
The idea that the Rega might be over-driving the Arcam is almost as odd, considering the fact that it's 2017 and "matching" levels should be unnecessary because the preamp output should be usable from any preamp, IMO. There's no good reason to have brand-specific output level and impedance at this time. That said, make sure these are compatible from one piece to another.
 
TLS Guy

TLS Guy

Seriously, I have no life.
I have just purchased a new Arcam P49 power amplifier from a UK retailer. As it was a store demo and I was able to deduct VAT, I got a fantastic deal. I've connected this to my Rega Elicit-R amplifier, which I'm using as a pre-amp, to drive a pair of KEF Q900s. The combination sounds really amazing. But when the Rega is set to 12 o'clock or higher, the P49 shuts off automatically.

Could somebody please explain to me why this is happening? Is there some type of gain mismatch going on? The retailer is now suggesting I buy a step down attenuator. Or did the retailer just send me a dud P49?

Thanks for helping me understand what's going on.

-Johnny
I assume you have this connected via the RCA pre outs of the Rega. There should be no mismatch.

The first thing to do is make absolutely sure there are no stray wires in the speaker connection. Just the smallest stray wire will cause this. So job one is to carefully redo the speaker connections.

Now even though the Arcam is a powerful amp, it may not like those speakers.

The speaker's impedance is specified as 8 ohms, but as fig.1 shows, the impedance drops to 4 ohms in the top octaves and to below 4 ohms in the lower midrange, reaching a minimum value of 3.17 ohms at 160Hz. There is also a combination of 5 ohms magnitude and –42° electrical phase angle at 80Hz, meaning that the KEF does need to be used with a good 4 ohm–rated amplifier or receiver.



I would classify that as a very difficult load calculated to upset quite a few amplifiers. Not only are these speakers NOT 8 ohm, but below 4 and the negative phase angles in the low impedance region make these speakers hard on amplifiers.

The review can be found here.

The only other possibility is that the Arcam amp is defective.

The only way to sort this out is to drive the amp with speakers presenting a less adverse load.

In summary, the Arcam is either defective or the mismatch is the speaker/amp interface, assuming there is no speaker wiring problem which should be thoroughly investigated first.
 
P

PENG

Audioholic Slumlord
The idea that the Rega might be over-driving the Arcam is almost as odd, considering the fact that it's 2017 and "matching" levels should be unnecessary because the preamp output should be usable from any preamp, IMO. There's no good reason to have brand-specific output level and impedance at this time. That said, make sure these are compatible from one piece to another.
I meant the Arcam amp is probably just over driven by the owner:D, and the speakers, not the Rega preamp outputs. As I also said, the retailer's suggestion of using an attenuator makes no sense. Of course the unit could be defective too as TLSG mentioned, though that would be highly unlikely, as it is brand new unit and is an Arcam.
 
TLS Guy

TLS Guy

Seriously, I have no life.
I meant the Arcam amp is probably just over driven by the owner:D, and the speakers, not the Rega preamp outputs. As I also said, the retailer's suggestion of using an attenuator makes no sense. Of course the unit could be defective too as TLSG mentioned, though that would be highly unlikely, as it is brand new unit and is an Arcam.
I agree it probably is over driven because of the adverse speaker load.

This is he spec of that Arcam.
  • Both channels driven, 8Ω,20Hz-20kHz, 0.2% THD - 200W
  • Single, 4Ω, at 1kHz - 400W
Note there is only a one channel driven spec into 4 ohm. That is telling you this amp is current limited into a four ohm load driving 4 ohm speakers. These speakers are less then four ohms.
They are 3.17 ohms at 160 Hz and when you allow for the -42 degree phase angle at 80 Hz the amp sees 3.5 ohms. So these speakers are very low impedance where power demands are high and will embarrass a lot of amps, especially if the 4 ohm spec is rated at only one channel driven.

People really need to be wary of speakers with more than two bass drivers. They are just inviting this sort of thing.

One thing for sure, if he keeps sending it into reset, if it not defective now it soon will be!
 
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P

PENG

Audioholic Slumlord
I assume you have this connected via the RCA pre outs of the Rega. There should be no mismatch.

The first thing to do is make absolutely sure there are no stray wires in the speaker connection. Just the smallest stray wire will cause this. So job one is to carefully redo the speaker connections.

Now even though the Arcam is a powerful amp, it may not like those speakers.

The speaker's impedance is specified as 8 ohms, but as fig.1 shows, the impedance drops to 4 ohms in the top octaves and to below 4 ohms in the lower midrange, reaching a minimum value of 3.17 ohms at 160Hz. There is also a combination of 5 ohms magnitude and –42° electrical phase angle at 80Hz, meaning that the KEF does need to be used with a good 4 ohm–rated amplifier or receiver.



I would classify that as a very difficult load calculated to upset quite a few amplifiers. Not only are these speakers NOT 8 ohm, but below 4 and the negative phase angles in the low impedance region make these speakers hard on amplifiers.

The review can be found here.

The only other possibility is that the Arcam amp is defective.

The only way to sort this out is to drive the amp with speakers presenting a less adverse load.

In summary, the Arcam is either defective or the mismatch is the speaker/amp interface, assuming there is no speaker wiring problem which should be thoroughly investigated first.
Cosine 40 degrees is 0.766, I agree that is not good, but not really that bad for amps as powerful as the Arcam P49, even if it coincides briefly with low impedance. I have the KEF R900 that are as "difficult" as the Q900 and I have no trouble driving them with my AVR-3805.

I think the OP might have just crank the volume up too high (it's all relative right..) and overloaded the amp, that's why I asked the questions I asked in my post. I also won't rule out the "defective" possibility, but it's an Arcam, and brand new, so the odds for that should be low.
 
AcuDefTechGuy

AcuDefTechGuy

Audioholic Jedi
What happens if you just use the Rega Integrated Amp without using the Arcam Amp?

Does the Rega Integrated Amp also shutdown when you reach the same volume?
 
highfigh

highfigh

Seriously, I have no life.
I meant the Arcam amp is probably just over driven by the owner:D, and the speakers, not the Rega preamp outputs. As I also said, the retailer's suggestion of using an attenuator makes no sense. Of course the unit could be defective too as TLSG mentioned, though that would be highly unlikely, as it is brand new unit and is an Arcam.
There's no good reason its volume control's taper should reach that output level with the volume control at the 12:00 position.

After as long as I have been in the audio business, it's not impossible that it's defective, but if the output of the Rega is so hot that the Arcam can't handle it, I have to wonder what's going through the heads of the designers at Rega. If they make products that aren't compatible with most others, THEY'RE at fault, not Arcam for making amplifiers that can't handle the output of the preamp. If the Arcam really can't handle the load, I have to wonder why they didn't consider difficult loads in the design phase.
 
P

PENG

Audioholic Slumlord
There's no good reason its volume control's taper should reach that output level with the volume control at the 12:00 position.
You can't just go by the volume position. However, if we are talking 10 O'clock, I can agree with you. At 12, depending on the source (example: cd player), player output level, the recording, and the gain structure, it is possible to drive a power amp with 31 dB gain to level well beyond it's rated output. We really don't have all the facts to say anything definitive. At the minimum, It will help if we know the spl the op is getting at a given distance, when the volume is at 12, before the amp shuts down.
 
mtrycrafts

mtrycrafts

Seriously, I have no life.
Also, the OP indicated it was a demo unit that he got a good deal on. Who knows how long it was a demo.
 
highfigh

highfigh

Seriously, I have no life.
You can't just go by the volume position. However, if we are talking 10 O'clock, I can agree with you. At 12, depending on the source (example: cd player), player output level, the recording, and the gain structure, it is possible to drive a power amp with 31 dB gain to level well beyond it's rated output. We really don't have all the facts to say anything definitive. At the minimum, It will help if we know the spl the op is getting at a given distance, when the volume is at 12, before the amp shuts down.
What we really need to know is the power amp's sensitivity and the output voltage of the preamp and their input/output impedance. Beyond that, it's all just speculation. Knowing SPL is useless without knowing the others I mentioned.

When have you ever seen a power amp that reached full output when the preamp's volume control was at 10:00-12:00? I haven't, unless it was one very old piece and something much newer, which have different needs WRT voltage and impedance.
 
lovinthehd

lovinthehd

Audioholic Jedi
Goofy set up to add an amp to an amp somewhat especially without any idea why it was thought a good plan in the first place....
 
AcuDefTechGuy

AcuDefTechGuy

Audioholic Jedi
Goofy set up to add an amp to an amp somewhat especially without any idea why it was thought a good plan in the first place....
That's what I'm thinking also. That's why I asked if he had tried using just one amp (the Integrated Amp), instead of using both. :D
 
P

PENG

Audioholic Slumlord
What we really need to know is the power amp's sensitivity and the output voltage of the preamp and their input/output impedance. Beyond that, it's all just speculation. Knowing SPL is useless without knowing the others I mentioned.

When have you ever seen a power amp that reached full output when the preamp's volume control was at 10:00-12:00? I haven't, unless it was one very old piece and something much newer, which have different needs WRT voltage and impedance.
For you to post something like this is really unusual and I am really surprised:). I respectfully (hopefully that goes both ways) disagree with you. I read the specs/manuals of both his Rega and ARCAM amps before I posted, so I had a good idea of their specs, including input/output impedance, sensitivities, and gains as well, but did you? If you did read the manual of the P49, I hope you did not miss the fact that the ARCAM has a gain selector for 25 or 31 dB, another thing to consider.

Knowing just the sensitivities and gains of the amps are not enough because there are other factors in the chain, and I cited examples, if you care to read my previous post before you respond.

If we know the spl at his listening position and at the point near shutdown, then since we already know the speaker's characteristics including their sensitivities, we can calculate approximately the output level of the amp at volume 12 from his sitting position; and determine if that exceeded the amp's rated output and by how much. That is not speculation and not "useless" if you understand, and I am sure you do, the relationship between amp power output, and spl at distance. For sure you know how spl calculators (example: http://myhometheater.homestead.com/splcalculator.html) works.

I have experience with quite a few gear including avr, prepros, preamp, amps, integrated amps so I know it is not unusual for the power amp to reach full output when the preamp's volume control is at 12. I can actually demo this in at least one of my two channel system. Again, without knowing the information we need, we are speculating and I stated that in my post so you are repeating something I stated, that means we agree on this one point.

I have never said it could happen at 10:00, in fact I specifically said at 10:00, I would agree with your point. So please, before you respond again read my post to avoid misquoting me.
 
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highfigh

highfigh

Seriously, I have no life.
For you to post something like this is really unusual and I am really surprised:). I respectfully (hopefully that goes both ways) disagree with you. I read the specs/manuals of both his Rega and ARCAM amps before I posted, so I had a good idea of their specs, including input/output impedance, sensitivities, and gains as well, but did you? If you did read the manual of the P49, I hope you did not miss the fact that the ARCAM has a gain selector for 25 or 31 dB, another thing to consider.

Knowing just the sensitivities and gains of the amps are not enough because there are other factors in the chain, and I cited examples if you cared to read my post before you respond.

If we know the spl at his listening position and at the point near shutdown, then since we already know the speaker's characteristics including their sensitivities, we can calculate approximately the output level of the amp at volume 12 from his sitting position; and determine if that exceeded the amp's rated output and by how much. That is not speculation and not "useless" if you understand, and I am sure you do, the relationship between amp power output, and spl at distance. For sure you know how spl calculators (example: http://myhometheater.homestead.com/splcalculator.html) works.

I have experience with quite a few gear including avr, prepros, preamp, amps, integrated amps so I know it is not unusual for the power amp to reach full output when the preamp's volume control is at 12. I can actually demo this in at least one of my two channel system.

I have never said it could happen at 10:00, in fact I specifically said at 10:10, I would agree with you so please, before you respond again read my post and do not put word in my mouth.
You didn't say it could happen at 10:00, your comment included the word 'if'.

I don't remember seeing any mention of how long the system had been operating before it shuts down- this might be helpful.

The specs for the Rega show "Pre-amplifier output - 760mV for rated inputs" and that's lower than many amplifiers need in order to achieve this. The Arcam specs state "Nominal sensitivity for 200W / 8Ω - 1.15V (RCA), 2.3V (XLR)". I looked at the Parasound A23's specs for sensitivity and it shows "Input sensitivity:
1 V for 28.28 V, THX Reference Level".

Is it possible that the input would be exceeding the sensitivity of the Arcam? I doubt it.

If you can demonstrate the maximum output at 12:00, are you using an oscilloscope? If not, I wouldn't recommend using a multi-meter because they usually don't respond as fast as it needs to. How would you demonstrate this?

How, exactly, would the max output exceed spec? The Kef Q900 spec shows 91dB/2.82V, so 31dB of gain, using the Arcam and Rega specs to set up the equipment to achieve their max output, comes to 122dB without adding/subtracting anything due to the room's acoustics. He didn't mention anything about SPL or the output being extremely loud, only volume control position.

Input/output specs are used when designing an audio system and the sensitivity vs output is one place where many system fails to perform, whether it's because of excessive distortion, lower output than needed or damaged speakers. The desired SPL is part of the list of design criteria, but measuring that in order to determine why the amp shuts is not a typical method. The first thing a service tech would do is connect the amp to load resistors and a scope, then connect it to a source that can feed it what it needs to SEE when it starts to distort. A test bench isn't likely to have a load that can replicate what these speakers present, so they would need to have them in order to find the fault.

Please explain your thinking WRT measuring the SPL as a way to find the problem, other than finding that it's lower than stated. Knowing that the amp uses Class G, my guess is that the power supply has some kind of problem.
 
P

PENG

Audioholic Slumlord
I don't remember seeing any mention of how long the system had been operating before it shuts down- this might be helpful.
I agree with you 100% on this.

The specs for the Rega show "Pre-amplifier output - 760mV for rated inputs" and that's lower than many amplifiers need in order to achieve this.
Not necessarily as it depends, it would appear that Rega emphasized the 760 mV figure because it's build in power amp has a gain of 31.5 dB, that means at 760 mV input, the power amp section will be at rated output of around 105W. It is lower than many amps mainly because it's relatively high gain (31.5 Vs THX's 29 dB) and lower output level.

Is it possible that the input would be exceeding the sensitivity of the Arcam? I doubt it.
No idea why your would "doubt it".. Of course the input can exceed the sensitivity of the Arcam P49. As you know the "nominal" sensitivity of the P49 is only 1.15V. Even the recently tested AVR-X3300W can output 4.5V unclipped.

http://www.audioholics.com/av-receiver-reviews/denon-avr-x3300w-1/measurements

As Gene said: "A preamp output of less than 2Vrms is unacceptable in my book."

Rega did not specify it's maximum pre-amp output level, but I sure hope a relatively expensive unit like that can output more than 2V, and that would have no trouble driving the Arcam to exceed it's rated output. Note: the 760 mV was not stated as the "maximum" output, it was simply stated as the level when the Rega will be at rated output.

If you can demonstrate the maximum output at 12:00, are you using an oscilloscope? If not, I wouldn't recommend using a multi-meter because they usually don't respond as fast as it needs to. How would you demonstrate this?
No need for an oscilloscope for this, I will still use my Fluke 87V multi-meter even if I have a good scope on hand. First of all, I wasn't measuring peak, but true RMS V max and average only. There was no need to, but I actually always measure peaks as well just for knowledge, the 87V can measure transients as fast as 250 micro seconds, not that fast but fast none the less. You can see the features/specs of my mm in the link below:

http://en-us.fluke.com/products/digital-multimeters/fluke-87v-digital-multimeter.html#features

By the way, one of my power amp, the Marantz SM-7, has analog watt meters. I don't want to risk smoking it, otherwise I could do a demo and show you the photos.

How would you demonstrate this?
As an example, I would simply hook up my Denon DVD3910 that has output voltage of approx. 2.5V or higher depending on the source CD, DVDA, SACD etc. That should be high enough to drive one of my preamp to output >2V with vol at 12.

How, exactly, would the max output exceed spec? The Kef Q900 spec shows 91dB/2.82V, so 31dB of gain, using the Arcam and Rega specs to set up the equipment to achieve their max output, comes to 122dB without adding/subtracting anything due to the room's acoustics. He didn't mention anything about SPL or the output being extremely loud, only volume control position.
I understand what you are getting at. The OP would have to risk hearing damage to drive his Rega/Arcam setup to trigger a thermal shutdown or high current (e.g. short circuit) protection shutdown, but the point is, it is possible. Let's say he has a large room of 20'WX30'LX15'H and he sits 16'X20' from the speakers. Now you do the math or use the more conservative Crownaudio spl calculator, you will see that it is not hard to get the Arcam to exceed it's 200 WPC into 8 ohm, both channel driven or 400 WPC into 4 ohm, one single channel driven ratings.

Keep in mind as TLS Guy pointed out, the speakers low impedance level coincided with a relatively high phase angle. That alone should not be an issue, but if all the stars align, and if the OP listen at above reference level, say 90-95 dB at 16 to 20' from the speakers, then things could get ugly.

I only suggested measuring the spl from a given distance so we could rule out the possibility that the OP may be listening way too loud sitting far away in a large room (if he is in a large room). It is so easy to do to rule this unlikely scenario out so why not?

Please explain your thinking WRT measuring the SPL as a way to find the problem, other than finding that it's lower than stated. Knowing that the amp uses Class G, my guess is that the power supply has some kind of problem.
So you misunderstood me all along. I am not suggesting this as a way to find the problem. I am suggesting doing it to rule out a potential probable cause. Again, it is easy to first rule out the listening too loud while sitting too far, and driving speakers with impedance dips over a range that coincides with a somewhat difficult phase angle etc etc scenario. After proving this to be negative, then the OP should be on firmer ground to ask for an exchange. I hope I made it clear this time as I think it is time to move on.

Edit: for calculation purposes, I would use not use the 91 dB/1W/1M for sensitivity because it is specified on /2.83V/1M basis.. 89 dB/1W/1M would likely be more realistic because looking at the graphs TLSGuy posted, the 8 ohm nominal spec seems too optimistic. I disagree with TLSG's 4 ohm assessment but I would go as low as 5 to 6 ohms for power calculations or online calculators that are based on dB/1W/1M sensitivity specs.
 
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highfigh

highfigh

Seriously, I have no life.
I agree with you 100% on this.



Not necessarily as it depends, it would appear that Rega emphasized the 760 mV figure because it's build in power amp has a gain of 31.5 dB, that means at 760 mV input, the power amp section will be at rated output of around 105W. It is lower than many amps mainly because it's relatively high gain (31.5 Vs THX's 29 dB) and lower output level.



No idea why your would "doubt it".. Of course the input can exceed the sensitivity of the Arcam P49. As you know the "nominal" sensitivity of the P49 is only 1.15V. Even the recently tested AVR-X3300W can output 4.5V unclipped.

http://www.audioholics.com/av-receiver-reviews/denon-avr-x3300w-1/measurements

As Gene said: "A preamp output of less than 2Vrms is unacceptable in my book."

Rega did not specify it's maximum pre-amp output level, but I sure hope a relatively expensive unit like that can output more than 2V, and that would have no trouble driving the Arcam to exceed it's rated output. Note: the 760 mV was not stated as the "maximum" output, it was simply stated as the level when the Rega will be at rated output.



No need for an oscilloscope for this, I will still use my Fluke 87V multi-meter even if I have a good scope on hand. First of all, I wasn't measuring peak, but true RMS V max and average only. There was no need to, but I actually always measure peaks as well just for knowledge, the 87V can measure transients as fast as 250 micro seconds, not that fast but fast none the less. You can see the features/specs of my mm in the link below:

http://en-us.fluke.com/products/digital-multimeters/fluke-87v-digital-multimeter.html#features

By the way, one of my power amp, the Marantz SM-7, has analog watt meters. I don't want to risk smoking it, otherwise I could do a demo and show you the photos.



As an example, I would simply hook up my Denon DVD3910 that has output voltage of approx. 2.5V or higher depending on the source CD, DVDA, SACD etc. That should be high enough to drive one of my preamp to output >2V with vol at 12.



I understand what you are getting at. The OP would have to risk hearing damage to drive his Rega/Arcam setup to trigger a thermal shutdown or high current (e.g. short circuit) protection shutdown, but the point is, it is possible. Let's say he has a large room of 20'WX30'LX15'H and he sits 16'X20' from the speakers. Now you do the math or use the more conservative Crownaudio spl calculator, you will see that it is not hard to get the Arcam to exceed it's 200 WPC into 8 ohm, both channel driven or 400 WPC into 4 ohm, one single channel driven ratings.

Keep in mind as TLS Guy pointed out, the speakers low impedance level coincided with a relatively high phase angle. That alone should not be an issue, but if all the stars align, and if the OP listen at above reference level, say 90-95 dB at 16 to 20' from the speakers, then things could get ugly.

I only suggested measuring the spl from a given distance so we could rule out the possibility that the OP may be listening way too loud sitting far away in a large room (if he is in a large room). It is so easy to do to rule this unlikely scenario out so why not?



So you misunderstood me all along. I am not suggesting this as a way to find the problem. I am suggesting doing it to rule out a potential probable cause. Again, it is easy to first rule out the listening too loud while sitting too far, and driving speakers with impedance dips over a range that coincides with a somewhat difficult phase angle etc etc scenario. After proving this to be negative, then the OP should be on firmer ground to ask for an exchange. I hope I made it clear this time as I think it is time to move on.

Edit: for calculation purposes, I would use not use the 91 dB/1W/1M for sensitivity because it is specified on /2.83V/1M basis.. 89 dB/1W/1M would likely be more realistic because looking at the graphs TLSGuy posted, the 8 ohm nominal spec seems too optimistic. I disagree with TLSG's 4 ohm assessment but I would go as low as 5 to 6 ohms for power calculations or online calculators that are based on dB/1W/1M sensitivity specs.
I wrote "I doubt it" before I saw the specs- I stand by my other comments and if the Rega can deliver 2V, which is still far more than the Arcam needs OR wants to experience, not only will the Arcam run out of headroom, it will sound like crap.

This may be a good example of three marketing departments whose creative writing skills combined to cause a group of components to not work as a true 'system'.

I would also have the amp checked for DC offset.
 
Bookmark

Bookmark

Full Audioholic
According to the Rega manual -

The volume control has a resolution of 1dB per step, giving a total range of 80 steps over the available gain or volume range of 80dB. There is a calibrated LED display comprised of 20 LED's, which indicates the gain level or relative position of the volume control; this is calibrated in 4dB steps, across the total gain of the volume range.

Given the arrangement of the front panel leds then 12 O'clock should equate to roughly 50%.

@Johnny
Which Rega input is being used? If input 1 is used then the Switch set to Line Level?
The Pre Amp output is being used to connect to the P49? There appears to be 2 other outputs?
Is the Rega capable of driving the Kefs? Can the volume be raised above 12 O'clock in such an arrangement?
Which position is the Arcam Mode switch set? Stereo, Bridged or Dual Mono?
Is the Arcam input set to XLR or RCA?
Is the gain on the Arcam set to 25 or 31?

Without all or at least some of these answers I'm not sure the others will be able to help.

From the P49 manual
Bridged mode - P49 only
In bridged mode only the L+ and R+ outputs of SPEAKER
1 are active.
WARNING: Do not make any connections to the other speaker
terminals, doing so will severely damage your amplifier.
 
P

PENG

Audioholic Slumlord
According to the Rega manual -

The volume control has a resolution of 1dB per step, giving a total range of 80 steps over the available gain or volume range of 80dB. There is a calibrated LED display comprised of 20 LED's, which indicates the gain level or relative position of the volume control; this is calibrated in 4dB steps, across the total gain of the volume range.

Given the arrangement of the front panel leds then 12 O'clock should equate to roughly 50%.

@Johnny
Which Rega input is being used? If input 1 is used then the Switch set to Line Level?
The Pre Amp output is being used to connect to the P49? There appears to be 2 other outputs?
Is the Rega capable of driving the Kefs? Can the volume be raised above 12 O'clock in such an arrangement?
Which position is the Arcam Mode switch set? Stereo, Bridged or Dual Mono?
Is the Arcam input set to XLR or RCA?
Is the gain on the Arcam set to 25 or 31?

Without all or at least some of these answers I'm not sure the others will be able to help.

From the P49 manual
Bridged mode - P49 only
In bridged mode only the L+ and R+ outputs of SPEAKER
1 are active.
WARNING: Do not make any connections to the other speaker
terminals, doing so will severely damage your amplifier.
Many questions have been asked before, including those you are asking now. Please also keep in mind the volume position is not the only thing we have to consider, though we can all agree at 12 O'clock it typically (but not always) shouldn't present a problem, but it still could. The input level, that is, the output level of the media source also has to be considered. For example, if the input to the Rega is 2V, you only need a gain of 3 dB to output 4V, and that would drive the P49 well beyond it's clipping point. Conversely, if the input is 0.316V, you need 9 dB of voltage gain to get an output of 2.56V. That's how some people get away with, or actually prefer passive preamps.

https://www.kfs.oeaw.ac.at/manual/3.8/html/userguide/461.htm
http://en.wikiaudio.org/Line_level#Line_level_in_traditional_signal_paths
https://www.stereophile.com/content/music-first-audio-baby-reference-passive-preamplifier

I have at least one universal player that has output level of about 2.5V rms depending on the CD's recording, higher in some cases if the source file is downloaded from you-tube in MP3. So whenever my preamp is selected to that player, I have to dial the volume way down.
 

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