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Don Nebel 28th May 2006 12:57 AM

Dynaco preamp rebuild
 
Hello, I am new to this hobby and I have 3 preamps to play with. I have a Dynaco PAT 4, Dynaco Pat 5 Bi- Fet, and a Hafler DH100 Series II. In stock form the Hafler is the best sounding. I am considering first to rebuild the PAT 4, mainly because of the apparent simplicity of the circuit. The PAT 4 has only 4 transistors in each channel. My plan is to remove the tone controls and replace all the passive components with higher quality parts. This preamp has Electrolytic capacitors in the signal path. I was thinking of Black Gates to replace them. Now the active parts are outdated as well. The preamp uses (2) BC109B transistors in the phono section and (1) BC109B and (1) 2N3053 transistor in the output section. The manual specifies that the BC109B specs. are 240-500 Beta@5v,2ma. and the 2N3055 specs are 200-500 Beta@10v, 150ma. Can the performance of this preamp be improved with modern transistors, or is it a hopeless case. The Hafler uses NE5532 opamps throughout. I have some NE5535 opamps that have been biased for class A operation. I have considered these for the Hafler but I am sure about them for the phono section.The Hafler uses 5532's pre and post RIAA, in the tone section and output. Any thoughts about the possibilities? Thanks, Don Nebel

audiohead 28th May 2006 03:28 AM

Hi there:
Do not bother with either the dynacos or the Hafler.

Sonic performance is a strictly a function of circuit topology not designer parts/components.
regards,
art

chewrock 28th May 2006 05:07 AM

Hard Classics to Rebuild
 
These are some difficult old classics to rebuild.

The PAT-4 has very special tone controls that I'm not sure can even BE replaced. The wipers are non-standard configuration, as I recall. Maybe I'm thinking of the PAS-3x, which was the tube preamp from Dynaco.

Opinions differ about the PAT-5. Some "golden ears" people think that they are terrible. Some think that they are wonderful. I used to have a PAT-5. It ran my Stereo-70 just fine, but that was a tube amp that smoothed some of the roughness out of the sound.

OK, so here's the deal. You can swap out the capacitors and replace the resistors with low-noise carbon film. But that's not the big hurdle you face. The new transistors in the market place have hFE figures that are much too high for these designs. Yes, the noise would be lower, but the old values for the bias resistors in the original design will have the new transistors saturated all the time. They may not work at all! You could try to select some new transistors with lower enough hFEs to be drop in replacements, but what good would that do? You would have roughly the same noise and linearity that you've got right now. The whole point would be to find low-noise, high-speed, and high hFE replacements so that you could hear some real benefit for all this trouble.

In order to swap out the transistors, you would have to totally recalculate the bias for every stage, which might not be such a big deal, seeing as how you only have a few stages to calculate.

If you want to face that challenge, remember that collector current is roughly the bias current multiplied by the hFE. That means that you can "reverse engineer" the preamp stages. Disconnect the collector lead from one of the transistors and run the current through a top-quality milliammeter. Any of the new engineering grade Fluke meters should be accurate enough. Be sure nothing is shorted and then turn on the preamp. Write down the current for that stage.

Turn the preamp off. Reconnect the collector of that transistor.

Next, measure the live circuit voltages across the bias resistors for that transistor. Now, read the value of the bias resistors. Calculate the current flow into the base of the transistor using Ohm's Law.

OK, use your hFE meter to determine the beta gain of the low-noise transistors that you intend to use as replacements.

(You made sure to buy 10 times more transistors than you needed, right? You made sure to select a double hand matched set so that all the transistors in both channels would not only match each other, but match between channels, too, right? I mean, if you are serious about doing this, don't go halfway.)

Take the Collector current that you read from the circuit. Divide that by the hFE of your new transistor. That's the new bias current that you need. Reverse engineer the bias resistors to reduce the bias current down to the new value you just calculated. In most cases, and with the best new transistors, you will be looking at values in the 330K Ohm range and up. In one case, I had to buy precision 0.1% accuracy biomedical quality 510K Ohm resistors in order to get the collector current down to the original value without having the bias resistor noise increase.

Working one channel at a time, swap the original bias resistors with new metal film or bulk metal precision resistors with the new values. Then swap the old transistors with the new. Test the preamp after each complete swap of bias resistors and transistor. The first one that you do will be a sanity check on your calculations. If necessary, pull the collector lead on your new transistor and verify that you have the right collector current. The most common problem is that the Ohms Law calculations were a little off, and the collector current is usually too high. Increase the value of the bias resistor that goes to the top voltage rail and try it again.

Use the new values and swap the other channel out.

Now, go back and do the whole thing over again for the next gain section.

What can you expect? Well, the noise should drop significantly. The feedback and circuit topology determines the sound, as the man said. But the broader linearity of the new transistors should cause the sound on louder passages to seem less compressed. The replacement of the electrolytics should reduce some of the muddiness, but you might pay for it with some additional transistor type "edge", especially in the high treble ranges.

You might consider leaving the electrolytics in order to maintain a smoother tone. Here's a thought, get the best accuracy NON-polarized electrolytics you can find and use them, instead of the original polarized ones. Non-polarized electrolytics are specifically intended as feed-thru devices, and are generally better made for that application. You might get some reduction in distortion, while keeping the original "sound".

Finally, what to do about the Op Amps? I would take a serious look at the Burr-Brown OPA227-228 series (these are available in either single or double amp models), or maybe the OPA627-637 series (I think these are only available as singles). These are not cheap, but they do have vanishingly low distortion, quiet as a whisper, and as stable as a block of granite. Be prepared to spend $26 a piece for the 627s, about a third of that for the 227s. Make sure you read the data sheets carefully and get the right one for the right purpose. NewarkOne will let you download the TI data sheets for free.

Once again, opinions differ and some folks just don't think they are worth the money. But as I said, if you are going to do it, why go halfway?

Don Nebel 28th May 2006 01:38 PM

Thanks for the comments, I know that these preamps are ancient designs, but as the saying go's, they are paid for and if I break them I'm not out very much $$. Plus this is a learning experiance. I guess that now I have some homework to do. Don Nebel

warpedcorp 2nd September 2006 05:49 PM

Dynaco PAS Rebuild
 
I'm in the midst of rebuilding a dynaco PAS and have the following question:

The original circuit (Dynaco PAS Preamp) has a 250K volume control and a 750K balance control.

I'm thinking of replacing the original 250K volume control with the 250K alps I and then replacing the original 750K balance control with two individual 100K Alps pots to allow me to individually 'trim' each channel instead of a single balance control. Will this work or is the 750K value for balance a critical spec?

Thanks

masterchief 6th September 2006 08:37 PM

dynaco rebuild
 
750K pot on the balance most likely is not critical. I am assuming the high value is to be able to adjust balance more precisely. 2 100K should work, but if you are going to experiment ; I would try 250K pots to have a finer adjustment. Happy experimenting! Masterchief

anatech 6th September 2006 08:50 PM

Hi Don,
I built a PAT-5 back in the day. It didn't sound too good back then either, but it was for my High School AV Dept.

Those transistors are not too bad, replacement with newer ones should be easy. Don't forget they didn't match those parts.

You could use metal film in the signal path in the first stages. Metal Oxide or carbon film are fine everywhere else. Carbon resistors should be lined up and hit with a hammer in anything but high frequency circuits. The tone controls may as well be left in. You will have far too much gain if you bypass them.

Just replace the electrolytics with new ones, they are much better these days. You can use film in some spots. Work very hard to lose all ceramic capacitors in the audio circuits. Mica work very well.

You may actually end up with a much better sounding preamp. Not cutting edge, but better. Do one change at a time and listen to educate yourself. I think that is a great idea you have.

-Chris

anatech 6th September 2006 08:52 PM

Hi warpedcorp,
You could always just leave the balance control out. What's wrong with the original control anyway? You could also just leave it in for now too.

-Chris

chewrock 11th September 2006 03:38 AM

Hi Values for PAS are Needed
 
Hello,

I used to have a PAS-3x. The high values for the balance control are very necessary. This is a pure tube design that uses lower than normal voltage (and DC at that) for the cathode heaters.

This means that the current capability of the circuit is much, much lower than a modern day solid state preamp. Changing the balance control by a factor of 3 will increase the load on the prior circuitry by a factor of 9. That's because Watt's Law says that the square of the difference in resistance is the change in power. Lower the resistance by 3 times, increase the power dissipation by 9.

In sonic terms, this will cause the tone to become compressed sounding and "muddy". If you can't find a balance control at the original value, I would suggest putting 500K resistors in series with each grounded lead.

OK, I can hear the howls already. Putting two 500K ohm resistors in series with the ground leads will reduce the usable range of the balance control down to 2db, which is essentially useless for more than trimming the channels.

But it will preserve the original tone of the preamp.

anatech 11th September 2006 01:48 PM

Hi chewrock,
That's a pretty good suggestion. Who needs that range to begin with? Especially since we want a 0.2 dB channel match for gain or better.

-Chris


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