A blast from the past. An early Dynaco solid state system

[TLS Guy]

I'm just getting this vintage system ready for my eldest grandson. The speakers were built for my youngest daughter in 1993. I wanted to see how cheap I could make a decent pair of speakers. The total cost was $54.00 for the pair, selecting drivers from the Vifa classic range. They have a well balanced sound and excellent sound stage. Listening to them you would have no idea of their extreme budget cost. They are rear ported.

They have been to my eldest son, and now I have just got the system back form my youngest son, for my eldest grandson.

I have just finished getting the electronics up to snuff.

The preamp is the Dynaco, PAT 4-A

The power amp is the 60 watt per channel Dynaco 1204.

The tuner is a Dynaco FM 5.

The electronics was originally bought by my friend Phil when he was in Texas drafted for the Vietnam war.

I had to service an amp board for Phil many years ago, after he shorted the speaker wires. A little while later, the power supply failed, requiring major work.

When he later updated, he gave the rig to me, along with the original bill of sale!

Here are the pictures. I going to instal it for my grandson tomorrow.

I have been listening to the rig all afternoon. It is not fatiguing at all. I had forgotten how good the bass response is. They push well beyond their size!

Here are some pictures.

My grandsons TV has analog outs, so it will be AV. The preamp is very generous on the number of inputs!

I wonder how long this rig will be passed round the family?

 

[TheWarrior]

I love the idea of 'Family Heirloom Audio'.

Despite the frugality of the components, I need to ask about some of the cabinet construction choices. In my effort to better understand 'optimal' cabinet construction:

1) Why are the two driver's touching?

2) Why is the baffle surrounded on all 4 sides? Is it not supposed to be able radiate sound freely?

 

[TLS Guy]

I love the idea of 'Family Heirloom Audio'.

Despite the frugality of the components, I need to ask about some of the cabinet construction choices. In my effort to better understand 'optimal' cabinet construction:

1) Why are the two driver's touching?

2) Why is the baffle surrounded on all 4 sides? Is it not supposed to be able radiate sound freely?

You need to get the drivers as close together as possible to get the drivers as coherent as possible. This is the reason for the development of coaxial speakers.

The front edge of the cabinet is surrounded by 1/4 round flush with the baffle to avoid a hard edge. Cabinet edges are usually at 90 degrees or rounded, which reduces the magnitude of the diffracted wave, since the wave does not immediately expand into space upon reaching the edge. The benefits of edge rounding come into play only when the radius is greater than 1/8th wavelength. Thus a typical 1/2 inch radius begins to diffuse the diffracted wave at frequencies above 3.4 kHz, but will decrease in relevance at higher frequencies, when the driver illuminates less of the edge due to its increasing directivity.

 

[TheWarrior]

You need to get the drivers as close together as possible to get the drivers as coherent as possible. This is the reason for the development of coaxial speakers.

The front edge of the cabinet is surrounded by 1/4 round flush with the baffle to avoid a hard edge. Cabinet edges are usually at 90 degrees or rounded, which reduces the magnitude of the diffracted wave, since the wave does not immediately expand into space upon reaching the edge. The benefits of edge rounding come into play only when the radius is greater than 1/8th wavelength. Thus a typical 1/2 inch radius begins to diffuse the diffracted wave at frequencies above 3.4 kHz, but will decrease in relevance at higher frequencies, when the driver illuminates less of the edge due to its increasing directivity.

Excellent, thank you! Is there a correlation between the size of the radius vs. the thickness of the baffle being given the radius? And is there a noticeable difference between a 45 degree chamfer with a saw and a router/shaper cutter creating an actual radius?

I see a lot of DIY designs taking the 'cheaper' route of using a saw as opposed to the extra purchase of router bits. As a carpenter, the only difference to me is what sounds better!

 

[Swerd]

Is there a correlation between the size of the radius vs. the thickness of the baffle being given the radius? And is there a noticeable difference between a 45 degree chamfer with a saw and a router/shaper cutter creating an actual radius?

I see a lot of DIY designs taking the 'cheaper' route of using a saw as opposed to the extra purchase of router bits. As a carpenter, the only difference to me is what sounds better!

My understanding of this is similar to what TLS Guy said. In addition, the frequency at which diffraction peaks occur vary with the distance between the center of a driver and the cabinet edge. That's why you sometimes see tweeters mounted slightly off center. It produces two different edge-to-tweeter distances that generate two different diffraction peaks instead of one larger peak.

FYI, here is a comparison of the frequency response curves of two speakers where everything was identical except the cabinet edges. One had unmodified edges, and the other had rounded edges with ¾" radius.

http://murphyblaster.com/content.php?f=cabinets.html

This was done to demonstrate that smaller round overs, such as those with a ¾" radius could get the job done as well as really large round overs like 1½" radius.

I've never seen measurements from a cabinet with chamfered edges, but I suspect it will also be better than nothing. A lot of the variation you see among cabinets might depend on what tools and woodworking skills the builder has.

 

[TheWarrior]

Well yes, that comparison definitely confirms the need for rounding over! I've seen similar charts before, including a roundover on the hole for the drivers themselves, suggesting every surface on the baffle benefits from this treatment.

Good stuff. I am building an elaborate bookshelf design that I hope to complete end of the month. And then FINALLY I will have time for Thor cabinets!

 

[Swerd]

Well yes, that comparison definitely confirms the need for rounding over!

It only confirms that rounding over leads to better looking measurements. It is not clear if it's audible. Just the same, I agree with you. If I were building, I would take the time to round over the baffle edges.

I've seen similar charts before, including a roundover on the hole for the drivers themselves, suggesting every surface on the baffle benefits from this treatment.

Rounding over or chamfering the inside of the woofer hole of the front baffle is done only to allow more space for a driver to "breathe". The thicker the baffle, the more this might be needed. But I've never seen or heard a direct comparison.

This is done only for woofers that have open backs. Most tweeters come with closed backs, and any mid range (in a 3-way) should have either a closed back or be in a separate compartment from the woofer.

 

[TLS Guy]

My understanding of this is similar to what TLS Guy said. In addition, the frequency at which diffraction peaks occur vary with the distance between the center of a driver and the cabinet edge. That's why you sometimes see tweeters mounted slightly off center. It produces two different edge-to-tweeter distances that generate two different diffraction peaks instead of one larger peak.

FYI, here is a comparison of the frequency response curves of two speakers where everything was identical except the cabinet edges. One had unmodified edges, and the other had rounded edges with ¾" radius.

http://murphyblaster.com/content.php?f=cabinets.html

This was done to demonstrate that smaller round overs, such as those with a ¾" radius could get the job done as well as really large round overs like 1½" radius.

I've never seen measurements from a cabinet with chamfered edges, but I suspect it will also be better than nothing. A lot of the variation you see among cabinets might depend on what tools and woodworking skills the builder has.

The big limitation of all this is that we then put on a grill, and the grill frame does make this whole exercise somewhat pointless to a degree, unless you want to live with exposed drivers and eventually likely damage. In fact damage often occurs sooner rather than later.

I keep grills on and frankly can't hear the difference.

The round edges make the cabinet less likely to damage and look nice. The other issue is that it make the cabinets easier to build. You can use pre veneered board. You use a router blade.
The use the 1/4 round to cover the routs.

Here is a picture of one such cabinet.

All edges have quarter round. My two sons have a set of these. This is one of the speakers my youngest son has.

 

[Swerd]

The most difficult (for me), but the best looking way to round over the front baffle edges is to do it the way Jim Salk does.

Before assembling the cabinet, he makes the MDF front baffle smaller to accommodate ~1½" hardwood strips (usually maple) glued to the top and sides. See the mitered corners on top. Then he trims them to the proper dimensions, rounds them over, cuts the holes, and finally glues them onto the front of the cabinet.

The photo shows pepperwood burl veneer and maple side strips dyed to match. I've also seen really nice combinations of mahogany crotch veneer with straight grained mahogany side strips dyed a lighter color to contrast the darker veneer.

 

[TLS Guy]

Here is a picture of the rig installed for my grandson.

He is ecstatic about it, and so is his younger brother. My eldest grandson has always liked classical music and now his younger brother has taken up percussion. He is making great strides with the xylophone. He is suddenly taking a keen interest in symphony orchestras.

It seems this rig will get some heavy duty service.

The speakers small as they are do not cry out for a sub. This may become a summer project though.

 

[BoredSysAdmin]

He is ecstatic about it, and so is his younger brother. My eldest grandson has always liked classical music and now his younger brother has taken up percussion. He is making great strides with the xylophone. He is suddenly taking a keen interest in symphony orchestras.