Why Are There Electrolytic Capacitors On My Inputs?

mlucitt

mlucitt

Veteran and General Yakker
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Jacksonville, FL
What do you guys think of the way Great American Sound designed a phono stage circuit with electrolytic caps in the sound path? The circuit calls out (4) 68uF/3v caps, but my pre-amplifier has (4) 100uF/10v caps.
 

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I wonder if that was part of the mods from the folks with the sticker on the front??
 
Good thing this isn't Google's Usenet portal...if you used that word for transistor or transformer the ads would be for INTERESTING SITES.
 
laatsch55 said:
I wonder if that was part of the mods from the folks with the sticker on the front??
Click to expand...
The circled diodes 1N4148s D301 (310) and D304 (307) are missing as well as diodes labeled R 302 (309) and R 303 (308) and D305 and D306. I think THOSE were mods done by the GAS folks, they literally pulled every diode off that board except the 28V zener across the opamp. Also curiously, R329 2.2K is missing and in its place are (2) 39K resistors from pin 5/3 and pin 8 of the opamp to ground. But that circuit is only part of the servo loop.
 
laatsch55 said:
What were the GAS 112 trannies??
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GAS 112 is cross referenced to NPN 2N5088 in the spec/schematic, on the board they are 2N4401 and 2N5210.
GAS 113 is the PNP complement and it is spec'ed as 2N4250 but on the board I see 2N5087.
They could have gone to newer P/Ns due to end-of-life, can hardly find GAS 113 (2N4250s) anywhere, but I see GAS 112 (2N5088) at Mouser (21.5K in stock).
 
The 2N5088 is our bias trannie now.......should be plenty of those..
 
I like the ideal of 2N5087 and 2N5088 when small signal complementary pairs are called for. Should be pretty easy to match their hfe, when you buy 1000 of them.
 
Mark, remember the Longfellow method of matching?
 
laatsch55 said:
Mark, remember the Longfellow method of matching?
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I should remember something about Longfellow. He said:
Music is the universal language of mankind.
and
In character, in manner, in style, in all things, the supreme excellence is simplicity.

But you will have to refresh my memory on the method of matching, and possibly educate some of the newer forum members.
 
Correction. Fred Longworth out of SanDiego, AK member and poster. For small signal TO 92'S measure resistance across the BE, BC junctions. Chart and separate into piles where those 2 measurements match .
The closer the 2 per device are the greater the gain. The closer those measurements are to the others of the same junction the lower the offset.
With 20 or so TIS 97's I was getting off sets of 4 or 5 mv...
 
Yes, I remember making those piles. Having a digital meter with alligator clips kept my sanity.

Sometimes you got lucky, especially if the transistors were in the same lot from Mouser. Too bad you can't sent the outliers back to Mouser to trade for some more stock that might be closer to the mean measurements.
 
The caps are needed because there may be a tiny DC voltage on the comp-diff pairs bases. Yes, the DC servo should keep DC close to zero, but a slight DC voltage will cause current to flow through the phono cartridge. It could ruin an MC, but wouldn't harm an MM. The change from 68uf to 100uf may have been to accommodate MC without rolloff.
 
nakdoc said:
The caps are needed because there may be a tiny DC voltage on the comp-diff pairs bases. Yes, the DC servo should keep DC close to zero, but a slight DC voltage will cause current to flow through the phono cartridge. It could ruin an MC, but wouldn't harm an MM. The change from 68uf to 100uf may have been to accommodate MC without rolloff.
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Agreed on the need for caps on the inputs, but I had never seen electrolytics used for this purpose. This amp is not rated for a MC cartridge, but I get your point, those things are sensitive and delicate. I am going to use 68uf Tantalum capacitors (four per channel), which of course are non-polarized. I don't have a turntable to test the phono section. I could send the pre-amp to my brother who has a nice Pioneer turntable with a Shure V15 Type IV cartridge and a newish needle.
 
If it were mine, I'd install 100uf 25 volt NP non-polar electrolytics. Tantalums are polarized, and are usually low voltage. What is needed here is low distortion, and any cap with a polarized electrolyte is going to make distortion. Adding a 0.1 uf poly cap in parallel won't hurt. Back when this preamp was popular, the rage was to run MC cartridges directly into 47k ohm phono stages. You had to turn them up, and the preamp had to be dead quiet, but better sound was usually obtained because one less device was in the signal path. The only thing that beat no head amp was a really excellent MC transformer.
 
mlucitt said:
I should remember something about Longfellow. He said:
Music is the universal language of mankind.
and
In character, in manner, in style, in all things, the supreme excellence is simplicity.

But you will have to refresh my memory on the method of matching, and possibly educate some of the newer forum members.
Click to expand...

Longfellow was that dude Neil Diamond sang about who had a serenade. Then again he sang about a supersonic seagull. Neil was trippy like that.
 
mlucitt said:
Agreed on the need for caps on the inputs, but I had never seen electrolytics used for this purpose. This amp is not rated for a MC cartridge, but I get your point, those things are sensitive and delicate. I am going to use 68uf Tantalum capacitors (four per channel), which of course are non-polarized. I don't have a turntable to test the phono section. I could send the pre-amp to my brother who has a nice Pioneer turntable with a Shure V15 Type IV cartridge and a newish needle.
Click to expand...

Hi Mark
I would be inclined to use something like this instead of a polarized cap of any sort. You want low leakage and electrolytic types have leakage.

810-FK11X5R0J686M
Or
810-FG20X5R0J107MRT6

Then again, I have not studied the schematic in too much detail. Looks like a servo'd output using the 1458...

Should think about C314 and C317 too...
 
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Gepetto said:
I would be inclined to use something like this instead of a polarized cap of any sort. You want low leakage and electrolytic types have leakage.

810-FK11X5R0J686M
Or
810-FG20X5R0J107MRT6
Click to expand...

Joe, that was my original thought, electrolytics are not ideal in this application. I looked at those TDK MLCC caps. They might be the better option than the polarized tantalum caps Lee and I originally agreed on.

Right now I have WIMA .1uf film caps installed because that has been my experience that they will block low-level DC sufficiently. Next I will try the tantalum caps that are enroute. If not satisfactory, I will use the MLCC caps that Joe suggests. Kind of a test where I hope the last option is the best.

NAKDOC makes a good point about small amounts of DC potentially on the complementary differential pair bases. They are spec'd as 2N5088/2N4250 but are actually 2N4401/2N5087.

The C314 pair is an interesting use of two electrolytic capacitors. I believe a back-to-back connection makes them appear non-polarized to the circuit. But is not 200uf a lot of capacitance across a servo IC like the 1458? I did replace them with newer caps of the same type.
 
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