On/off bump in complementary pair amp

[scottw]

Greetings,

I picked up a little orphaned amp at a thrift store to run my TV speakers- for everyday use. It turned out in need of main PSU caps, so I replaced those as well as the rest of the smaller caps in the amp. Bias was adjusted and is stable at ~ 70mA; DC offset is <50 mV and has no apparent adjustment.

The amp sounds good, for what it is, and is quiet. The only complaint I have is that is has about a 1/2 Hz turn on/off bump that you can see as the speaker cones deflect. I really haven't ever played around with transistor amps before and was wondering if this was possibly normal for a low-fi complementary pair amp?

thanks,

Scott

[clem_o]

Hi Scott,

For certain circuit topologies, definitely - has more to do with the input / voltage amplifier configuration than the output. A lot of solid-state amplifiers generally do have a thump or a click when powering up/down. Most manufacturers work around this by having a little speaker relay that disconnects the speakers at the proper times.


Cheers

[anatech]

Hi Scott,
Clem's right. In some amps the "little" relay is what's called a contactor (really big, high power relay).

You should see the really cheap single supply capacitor coupled transistor amps of the early 70's. They gave the speakers a mighty heave on power up or down.

-Chris

[clem_o]

Hi Chris,

Righto! But those contactors make as much noise as the speaker thump... !!! :-)


Scott,

You may want to drop the bias a bit - 70mA sounds a bit high for an amp serving 'TV' sound...

Cheers!

Clem

[anatech]

Hi Clem,
Thanks for reminding me. I don't know the model (would help here), but 70 mA sounds like it's on the very high side. A mosfet design maybe.

-Chris

[scottw]

Thanks for the replys.

No relays in my modest little amp. The bump doesn't seem to hurt little low wattage Foster speakers so I won't worry about it.

The only values I changed in the parts repaced was a 50% increase in uf for the main PS caps; and ~3X increase in uf for the caps filtering the frontend/input transistors. Was worried the larger caps for the frontend were holding a voltage longer and causing the bump.

As for the bias, I think you are right. Let it run several hours at that setting and the sinks were getting warmer then I thought they would. Gonna try about 40mA? As it came, left side was 105mA and right was 35mA and noticably cooler. Is that what they mean by "Tokyo drift"?

I think the outputs are just garden variety BJT's: SK A1102/C2577 which cross to 2SA1102 and 2SC2577 (NTE 37 and 36)

http://www.nteinc.com/specs/10to99/NTE36.html


Scott

[clem_o]

To me, 40mA sounds about right... but Chris has a lot more experience with this... :-)

Cheers!

Clem

[anatech]

Hi Scott,
What brand and model is this thing you have (that's #1).

Do not even look at NTE, ECG or any other replacement brand for specs. Do not use those parts. Get the real data sheet and talk facts (to yourself at least).

Small receivers should run at 10 ~ 20 mA typically. Some brands run hotter than others. You want to run as cool as poosible without reducing the sound quality. Increasing the bias current does not always make it sound better, although you may think it does. No bias will sound bad.

I don't recommend increasing capacitor sizes often without increasing the rectifier diode current rating. This may also cause the transformer to run hotter as the peak currents are higher. Not really what you want to do.

Higher value bypass capacitors will have worse high frequency performance, therefore forcing you to bypass the larger values. It will also throw off the timing of power up / down. I have reduced the value of these caps from time to time to reduce turn on thump while making sure the turn off thump did not get worse.

-Chris

[scottw]

I'm back. Thanks for your continued help on this thing and feel free to bail at any time.

What brand and model is this thing you have (that's #1).

Embarrassingly, it's a Toshiba SB-M22. Has Elna boards marked "87 94V-0" so I'm guessing since it says "Made in Japan" it's likely 1987.

Do not even look at NTE, ECG or any other replacement brand for specs. Do not use those parts. Get the real data sheet and talk facts (to yourself at least)

Well, that's all my searches came up with for these transistors, other than they were also used in Pioneer amps (SX-40 and maybe others).

Small receivers should run at 10 ~ 20 mA typically. Some brands run hotter than others. You want to run as cool as poosible without reducing the sound quality. Increasing the bias current does not always make it sound better, although you may think it does. No bias will sound bad.

I'm fine with having the bias low for this application. The problem I was having was at the higher bias the current seemed to stablize better. At the lower bias even after it idled for a couple hours the current kept changing some. I got to wondering if the bias was really that sensitive to temp change, so while I was measuring I blew on the transistor/sink and current dropped quite a bit then slowly came back up. I was amazed it was that sensitive, but remember I know vertually nothing about transistors. I'll try it again and be more careful of air flow around the amp.

I don't recommend increasing capacitor sizes often without increasing the rectifier diode current rating. This may also cause the transformer to run hotter as the peak currents are higher. Not really what you want to do.

The original 3 amp diodes were replaced with MUR820's. You concern for the trannys temp is duly noted but I thought the increase from 4700uf to 6800uf was a relatively modest one.

Higher value bypass capacitors will have worse high frequency performance, therefore forcing you to bypass the larger values. It will also throw off the timing of power up / down. I have reduced the value of these caps from time to time to reduce turn on thump while making sure the turn off thump did not get worse.

The cap that decouples the PS to the front end transistors was increased from 100uf to 330uf (main psu through 82ohms series resistor to decoupling cap). I thought this might effect the power up/down. I thought about bypassing this cap but didn't. I figured a Black Gate this size may not need bypassing. I know, I know-he put Black Gates in a $5 amp?!


Scott

[anatech]

Hi Scott,
What's wrong with a Toshiba SB-M22? I've sold worse to people when I worked at Radio Shack (yeah, I was young).

Is the bias transistor in contact with the heatsink? It doesn't sound like it is.

What are the numbers on the output transistors, or do you have a schematic?

I normally do not increase the size of these caps without a specific reason, then I think about it. It's really up to you. Since you changed the rectifiers it should be okay.

-Chris