Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: bibi on May 21, 2011, 04:50:21 am
-
Hello,
I have an old transistor organ that like the title says, has an extremely noisy output. The problem is hiss-- it sounds like
the magnified hiss of a thousand carbon comp resistors. That was my first guess since it of 70's vintage and has carbon
resistors throughout. However after tracing around the circuit I found that the vast majority of the noise is generated at the collector of
the second transistor (the circuit uses two of these amps bass and "solo" hiss is similar on both) It could be the amps are just magnifying the noise I suppose.
My question relates to how to quiet this circuit down without building a new circuit altogether. I could go the obvious route of just adding a low pass filter to the output and be done with it, but the output circuit is so unusual to me that I'd like to figure it out and see if it can be improved some way. The way I see it there is some positive feedback from the second collector to the first emitter with the 10uF and 100k. This seemed to be the root of the problem but when I disconnected the 100k resistors the noise is still present.
I've also replaced the collector resistors on both transistors to no avail...
So, anyone willing to have a look and offer some advice I would be most appreciative.
-
After further thought and testing I think the trouble does lie with the carbon comp resistors. The 12 oscillator stages have at least 30-40 resistors per osc. so the cumulative effect is probably what is causing the problem.
That and perhaps the fact that the chassis has not a lick of shielding. (Unless wood counts!) Literally there is no metal to be found in the whole organ except on the front panel where the controls are located.
I'd still like to figure that amp output though...
-
> perhaps the fact that the chassis has not a lick of shielding.
Shielding is about buzzz.
There's huge 60Hz "signal" in any electrified room.
There's no significant source of random un-pitched hiss in rooms.
Shielding can also be checked by moving around. As you get closer to the fluorescent lamp the buzzz gets louder. Long cord in the back yard it gets less buzz, though still some from the internal power supply.
> way I see it there is some positive feedback from the second collector to the first emitter
No, negative feedback.
> after tracing around the circuit I found that the vast majority of the noise is generated at the collector of the second transistor
The circuit has target gain of 200 (100K/500). Hiss may be present but inaudible on your tracer at this stage's input, 200 times louder at output.
I'm very leery of an organ that needs gain of 200 at output. Electronic (not tonewheel) organ tone generators usually have huge outputs, several volts. Voicing and mixing adds loss, but 200X?
Take voltage measurements. It may just be sick.
The numbers are not adding-up (but I'm tired). Are you sure the 10K is not really 1K?
Take out Q1's emitter cap. Gain wil be 10. Do you get enough output?
Replace the 100K and 500 NFB resistors with carbon film.
If old and cheap, I'd suspect every and all electrolytics. Note voltages, then disconnect all four electros. DC voltages should not change; if they do, there is a leaker cap. That's just idle curiosity, I think you may as well replace them all with fresh. Try Q2 cap far larger, 100uFd or more.
Find the power supply bypass cap for this stage and replace it bigger. It is unlikely a wheezy B+ cap will induce hiss in this high-NFB stage, but what the heck.
Put ~~300pFd across the 100K NFB resistor. Yes, this is a low-pass, but elegant.
> The 12 oscillator stages have at least 30-40 resistors per osc
True, but BIG hiss is not usual for carbon-comp in high level stages. Normally you find ONE carbon-comp that has opened-up inside.... replace that, and hiss is back to reasonable. The problem is finding which one. And it IS odd to have similar hiss on two channels.
-
Thanks for your help PRR.
Per usual, you've helped identify the problem-- the hiss disappeared when the 500r resistors in the feedback path were replaced.
Why it is that both of them went bad is very strange to me.
In regards to your other questions, the schematic pasted on the inside of the organ and what appears in the circuit do actually add up:
The 10k on emitter1 is correct. As well, I had already replaced all the elcos and the power supply with an LM317.
Obviously I need to brush up on my feedback theory-- I was looking at the emitter of the first stage and the collector of the DC coupled stage as being in phase...
Now the organ is much better behaved and can actually be used on tour and in the studio like it was meant to do.
I really appreciate the assistance!