Transistor amp from scratch

[Newmarket]

This is obscure and arguably over engineering the design.

JR

Maybe l. But I'd still say it's a good idea to take any failure mode into account esp when it's relatively easy to implement countermeasures.

 

[Gus]

Yes, first of all I adjusted it for exactly half B+ but then when I wound it up it did not have symmetrical clipping so I readjusted it.
best
DaveP

IIRC I once looked at a older Mcintosh? (Not sure about this it was about 20 years ago, I think it was a Mcintosh because it had a glass panel) solid state amp.
The output drive stage was bootstrapped however it had a higher supply voltage for the stage.

Maybe that is why it was not symmetrical at 1/2B+.

 

[DaveP]

I haven't read every post. What is the intended use for this amp? Is it a learning project, duplicating an old design? Just for Fun?

The answesr to these questions are in the earlier posts.
best
DaveP

 

[DaveP]

The power Supply is now finished. 2 x 39V separate supplies.



The back view:-


Old School Front!


Best
DaveP

 

[Mr. Moscode]

The answesr to these questions are in the earlier posts.
best
DaveP

I read them. I get it. I'm sorry you've been hit with that disease.

Nicely constructed. Reminds me of my father's home brew HAM transmitter.

Suggestions: Bootstrap cap should be rated for 50V since your power supply voltage is greater than 35 volts. I suggest putting a diode in series with R9 to keep the bootstrap cap from discharging back into the supply on positive peaks. It's just a floating power supply. Film bypass caps everywhere.

How does it sound? Have you heard it?

 

[Gus]

This thread reminded me of this web site
https://sound-au.com/project243.htm

 

[Mr. Moscode]

Here's to the dear departed past . . .
-Dave Frishberg

 

[Audio1Man]

Hi Dave

I have some things you may want to review. Items 2 through 4 might be needed.

1. TR3 bias circuit should have a fixed resistor added to the pot so TR3 will not turn OFF. This could burn up the output devices as full cross conduction can happen.

2. Add Collector to Base cap so TR3 never can oscillate. Maybe 100pf?

3. Add cap across TR3, Collector to Emitter. Maybe 10uF?

4. May need Base stoppers to TR5 & TR6. Maybe 10-22 Ohms?

5. Your Power Supply will have excess HUM. The Rectifier + & - voltages need to go to big caps terminals, the output loads connect to the big cap terminals. The way you have them now the CHARGE & DISCHARGE currents share a common wire.

6. The FUSE wiring should be wired with AC HOT going to the back side of the fuse so when removing the fuse, you can’t get shocked, if the AC HOT is wired to the ring, you can get shocked. This is a requirment to meet /qualify to get UL/CSA and others.

Duke

 

[JohnRoberts]

Hi Dave


5. Your Power Supply will have excess HUM. The Rectifier + & - voltages need to go to big caps terminals, the output loads connect to the big cap terminals. The way you have them now the CHARGE & DISCHARGE currents share a common wire.

I don't recall seeing a PS in the schematic... think about all the high current path wires like low value resistors that will experience voltage drops from current flowing in them. Most of these voltage drops will be small and some can be ignored but take care that charging currents and speaker return currents don't corrupt signal reference nodes.

6. The FUSE wiring should be wired with AC HOT going to the back side of the fuse so when removing the fuse, you can’t get shocked, if the AC HOT is wired to the ring, you can get shocked.

Duke

spoken like somebody who might have experienced that..... that is how we learn.

The beauty of this forum is we can learn from other forum member's mistakes.

JR

 

[moamps]

2. Add Collector to Base cap so TR3 never can oscillate. Maybe 100pf?

Didn't you mean TR2?

3. Add cap across TR3, Collector to Base. Maybe 10uF?

Didn't you mean between collector and emitter?

@dave,

you have done a wonderful piece of engineering as always,
I've always been a fan of "steampunk" minimalist designs.

 

[user 137017]

6 The FUSE wiring should be wired with AC HOT going to the back side of the fuse so when removing the fuse, you can’t get shocked, if the AC HOT is wired to the ring, you can get shocked.

Duke

this is a thing many people forget about including myself. on occasion i have been taken by surprise with my own work,

 

[Audio1Man]

I have updated my post #88
3. Add cap across TR3, Collector to Emitter. Maybe 10uF?
6. The FUSE wiring should be wired with AC HOT going to the back side of the fuse so when removing the fuse, you can’t get shocked, if the AC HOT is wired to the ring, you can get shocked. This is a requirment to meet /qualify to get UL/CSA and others.

Duke

 

[Ike Zimbel]

this is a thing many people forget about including myself. on occasion i have been taken by surprise with my own work,

I have updated my post #88

6. The FUSE wiring should be wired with AC HOT going to the back side of the fuse so when removing the fuse, you can’t get shocked, if the AC HOT is wired to the ring, you can get shocked. This is a requirment to meet /qualify to get UL/CSA and others.

Duke

I have seen MANY original 1176's and possibly other UREI / UA products of that vintage with this issue. It's the first thing I look for when I open one up. End topic swerve...

 

[Rob Flinn]

I normally wire the fuse straight after the mains inlet before the switch. But, probably not so much of an issue if one doesn't live in the U.K

 

[ruffrecords]

I normally wire the fuse straight after the mains inlet before the switch. But, probably not so much of an issue if one doesn't live in the U.K

I normally use an IEC socket with a built in fuse. Meets UL and CE.

Cheers

Ian

 

[Rob Flinn]

I normally use an IEC socket with a built in fuse. Meets UL and CE.

Cheers

Ian

So do I, but on the rare ocassions I fit a seperate iec inlet and fuse holder the fuse is the first thing after the iec inlet.

 

[warpie]

Putting some film bypass capacitors across all the 'lytics should help it sound cleaner.

Can you please elaborate? Do you mean a film cap across all lytics, even the AC coupling ones like C1? I don't recall ever seen a schematic with a film cap across an AC coupling capacitor. Why would this improve the sound and what value should be chosen? Thanks

 

[DaveP]

6. The FUSE wiring should be wired with AC HOT going to the back side of the fuse so when removing the fuse, you can’t get shocked, if the AC HOT is wired to the ring, you can get shocked. This is a requirment to meet /qualify to get UL/CSA and others.

This has been done, thank you for the tip
DaveP

 

[DaveP]

I put the whole thing together after testing the amp and power supply separately OK.

When I turned it on there was smoke from R8 on one channel so that has burnt out a 2W 100 ohm Pot! This has served to remind me that I should stick to valves/tubes! My first guess is that the driver transistor has failed, but I welcome other suggestions.
best
DaveP

 

[DaveP]

5. Your Power Supply will have excess HUM. The Rectifier + & - voltages need to go to big caps terminals, the output loads connect to the big cap terminals. The way you have them now the CHARGE & DISCHARGE currents share a common wire.

I don't understand what you mean, the +and- from the rectifiers go to the 3300uF caps and then to the amps.

The earthing arrangement is like this layout:-

best
DaveP