My passive preamp project – DIY wins again!

It has been a while since I last took out my soldering iron.

In the last couple of years I have been listening to a Marantz PM8003 integrated Amp. But recently I was attracted to the idea of a passive preamp. (Just a volume control)

From a purist’s perspective, a passive preamp is better because less is more. Removing the active pre stage allows the signal to go directly to the power amp, which means less distortion and colouration.

Before investing more money on a DACT stepped attenuator, TVC and other passive solutions, I decided to test the above hypothesis on my system. So, $50 and 3 hours of my weekend later, I created a simple ALPS pot housing in an aluminium case. (See photos) Nothing much to write home about, simply an RCA socket with decent cable, and one set of input with one set of output. The size of the enclosure is slightly larger, which leaves some room to upgrade to aTVC or DACT attenuator

Evaluation:

The most striking improvement from this passive pre is the improved sound stage and transparency. The sound stage was no longer confined to my two speakers and the back wall, it was much wider and deeper; well exceeding my expectations.

The separation also improved noticeably. What used to be a group of musicians, can now easily be defined as an individual player, or rows of players from back to front. It made the whole experience more life like, with the same effect being attained from a well recorded musical. I could tell that each of the performers were moving on the stage as if they were right there.

Compared to the active pre amp it was very transparent There was better coherency which was probably due to less phase shift distortion. This quality made the playback of large scale music so much more enjoyable.

Not all good news

This passive pre is not without short comings. Lower frequency extension is not as good as the active, which is expected.

Not having a remote can be a pain for some people. My DIY pre certainly has no remote control, but it doesn’t concern me.

My passive pre only has one set of inputs. The additional input could be included by adding an input selection knob switch and a larger enclosure.

Due to the lack of active pre stage, I needed to turn up the volume by 20% to obtain the same SPL.

Is it worth it?

You bet! 3 hours and $50 is all it took for a result that would surely have cost me much more if I was to have bought a branded passive.

I may also look into a TVC passive which is suppose to perform better than my cheap ALPS pot.

Please feel free to share your passive pre experience and any suggestions you may have to improve my project

Not to rain on your parade but it's possible that your integrated amp's preamp is passive. If it has pre-outs, connect it to a power amp and turn the Marantz off. If you still hear the source and can use the volume, switching and tone controls, it's passive. If it's remote controllable and has solid state controls, never mind.

Less can be more- you may be able to restore some of the frequency bandwidth by installing a bypass cap on the volume control. As the frequency increases, the pot's resistance becomes more of a factor in the response- a small cap can limit its effect on this. Guitar players often have this done to their volume pot because as the volume is turned down, the tone gets sucked away and it sounds dull and crappy.

Some integrated amps are passive, but don't call attention to it. I have a Sony ES integrated amp from the '80s and had the preamp separated from the power amp, using the ext adapter jacks, so I could connect my Conrad Johnson power amp and found out that the controls are passive. After turning the Sony off, the music still played and other than some of the video circuitry, all of the switching and controls work when it's off.

I am certain my Marantz has an active pre stage. The sound is different to my DIY passive

Chatta said:

I am certain my Marantz has an active pre stage. The sound is different to my DIY passive

Click to expand...

It wouldn't need to be active to sound different. Yours isn't actually a preamp- it's a passive volume control, right? There's no amplification at all.

Passive attenuators are okay as long as you use low capacitance cables of the shortest possible length from the attenuator to amplifier.

But don't be fooled - a simple active pre-amp variable gain stage using a $0.35 op amp and a few dollars of additional parts will give you an absolutely transparent pre-amp - that in addition you do not have to worry about cable capacitance and using super short cable runs.

-Chris

thank you guys

WmAx said:

Passive attenuators are okay as long as you use low capacitance cables of the shortest possible length from the attenuator to amplifier.

But don't be fooled - a simple active pre-amp variable gain stage using a $0.35 op amp and a few dollars of additional parts will give you an absolutely transparent pre-amp - that in addition you do not have to worry about cable capacitance and using super short cable runs.

-Chris

Click to expand...

Thanks I will keep that in mind keeping the cable short. maybe time to DIY an IC

highfigh said:

Less can be more- you may be able to restore some of the frequency bandwidth by installing a bypass cap on the volume control. As the frequency increases, the pot's resistance becomes more of a factor in the response- a small cap can limit its effect on this. Guitar players often have this done to their volume pot because as the volume is turned down, the tone gets sucked away and it sounds dull and crappy.

Click to expand...

Thanks for the tip off. But seems contradicting to the "less is more" ideal. How can we restore bandwidth by adding another cap to the signal path? Please explain if you can I am not very good with all the theories.

PS: If I want to add the cap like you suggested what value and brand do you suggest?

Chatta said:

Thanks for the tip off. But seems contradicting to the "less is more" ideal. How can we restore bandwidth by adding another cap to the signal path? Please explain if you can I am not very good with all the theories.

PS: If I want to add the cap like you suggested what value and brand do you suggest?

Click to expand...

The cap & resistor bypass the volume control where the frequency response starts to be affected. Less is more but let's be realistic- a circuit with no components isn't possible and everything in the signal path will be subject to losses because of the components. That's part of the reason for active circuits- to counter losses and to buffer one stage from another.

Brand doesn't matter but the value will be determined by the impedance of the circuit.