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ouimetnick 9th September 2012 12:46 AM

Mod or rebuilt a Dynaco PAT-4?
So I have this Dynaco PAT-4 preamplifier. The boards are in tough shape, and its not the nicest sounding unit either. It doesn't sound terrible, but it isn't that good. The sound sounds smooshed together. The DH-101 that I actually use is MUCH better in sound. Highs are highs, mids are mids, and lows are lows. The bass knob on the PAT-4 seems to also adjust the mids too. :confused:

This wouldn't be for critical listening, but just to take the sound output of an iPod or my computer, and spit out the correct power to drive an amp. The iPod and computer at full volume don't put out probably half of the 100W my DH-200 can pump out. So thats why I use my DH-101 pre. Plus it has Bass and Treble controls.

Now this PAT-4 would do the same thing the DH-101 does, but be for my Peavey CS-800 amplifier. This is a pa amp, and is more about filling a large room with sound, and not so much for critical listening. I suppose you could say it is about quantity, not quality.

Anyways, back to the PAT-4. I have some copper clad boards, and can copy the layout, etch new PC-16 boards, and stuff it with quality components. 1% metal film resistors, better caps, replace the transistors with faster modern ones, turn the high frequency switch into a tone enabler/disabler. would be what I'm talking about as for some of these changes.

But I'm really wondering if I should bother. Would these changes turn it into a great sounding preamp. I'm sure it would change it for the better, but for the money for the parts, would it be dramatically better, or is the limit the actual circuit? When it isn't being used with the CS-800, I would use it for regular stereo duties. So I do want it to sound good.

Or is there a circuit I should build and stick it in the PAT-4 chassis, and it would be better in terms of money, and SOUND? One of the things I hate with the PAT-4 and the Stereo 120 is the components. Some of the capacitor values are just plain strange, and I can't find a replacement. Then the PAT-4 also uses strange goofy pots. They still are fine, except the volume pot is also borked.

Sorry for the long read.

djoffe 9th September 2012 01:27 PM

Perhaps I should recuse myself from this discussion, given that I have a vested interest of a minor nature. But with that disclosure behind me, here goes...

If you're only using the high level stages of the PAT-4, you end up with a pretty blameless path if you put in the DRD4 mod. That still leaves the tone controls, but of course, you can switch them out with the TCS. The only other suspicious thing in the signal path at that point is C10, a 1 uF 15 volt electrolytic (1 for each channel). There are some more electrolytics in the signal path, but they tend to be big enough to not cause problems. The advantage of this approach is a nice bump in the sound quality at a really low price. Of course, if you're going to replace all the old components, then "really low price" might not apply.

The high level stage is discrete and class A, which should be of some benefit. There are of course modern opamps that have marvelously low measured distortion. I have seen an Ebay seller with a pretty expensive set of replacement boards based on LM??? opamps. If memory serves, it was about $300. The mod did cut a lot of the functionality out of the preamp, but it's typically stuff that isn't that often used.

But, if you're really only interested in the high level stage, then maybe a few mods of the PAT-4 is really the path to take. AVA audio used to sell a mod kit (perhaps they still do). Qua-co, an Ebay seller, offers various cap kits for the power supply and the signal path. Anything that replaced old poly caps with new ones is probably a waste of money, but adding power supply capacitance, or re-doing the signal path electrolytics is probably a good thing.

About the funny value caps...that's true in the big electrolytics. You could replace them with the nearest standard value with no problem. E.g. 50 uF gets replaced by 47 uF with no issue.

Update My Dynaco

The PAT-4 Pages

ouimetnick 10th September 2012 03:33 AM

Thanks Dan. :) anyone have an opinion on what I should do?

kevinkr 10th September 2012 04:15 AM

I have a friend who built a tube pre-amp in his PAT-4.. :D Should be plenty of options here for solid state designs you could stuff into the existing space.

planet10 10th September 2012 04:40 AM

I'd consider a PAT4 a transformer, chassis & knobs.


ouimetnick 11th September 2012 12:44 AM

Any other ideas? An IC based preamp? Something besides the original circuitry.

kstagger 13th October 2012 12:58 PM

In a fit of silliness, I once actually updated a Dynaco PAT - new coupling caps, new PS caps, bypassed the tone controls, and removed some of the electrolytic in the tape and input section. The sound dramatically improved. Sure, it isn't a Mark Levinson - or whatever, but it no longer is an electronic embarrassment.
6th Street Bridge: Upgrading the Dynaco PAT-4 preamplifier

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