Problem with Harman Kardon Modified Clone

mshey904

Member
2020-03-10 7:35 pm
Hi,
I decided to make this amplifier from this page.
After assembling, I have problem with the sound.
DC Offset output is 14mV with +/- 37V.
The OpAmp is TL072 I replaced it many times.
Speaker is connected to before Zobel network.
The hfe of 2N3906 is 362 (max 300 in datasheet) and mpsa42/92 : 78/81 and mje15032/33 is 130/160.
Is it because of the hfe?

Thanks.:)

IMG_2409.jpg

The output sound
Picture of amp original quality
 

Anducer

Member
2011-03-21 3:44 pm
This looks like a good design to get an amp up and running quick.

I am a little concerned about the picture you presented as the testing phase---its seems almost every component should be well soldered on the top of the board were the leads enter the traces--are there any traces on other side or ground plane?
 

mshey904

Member
2020-03-10 7:35 pm
Where are the Power transistors? 2SC5200 & 2SA1943. All you have connected are the drivers.
I just have 2 pair of Toshiba transistors. I thought it should be okay if I use it in low sound. Is it effective even in low power and very low sound?

are there any traces on other side or ground plane?
Yes, I soldered all components on the other side.

Is this a board you purchased or made?
I made it, yeah because of VIAs I had to solder all of the components on the other side.
Thank you guys for helping.
IMG_2416.jpg IMG_2415.jpg
 
Last edited:

mshey904

Member
2020-03-10 7:35 pm
Hmm, 37V. Are you familiar with the threshholds for electrocution? I would rather poke around with an oscilloscope ;)

With the extended leads to the output devices can you be sure this is stable?

No, I'm not what is that sir?
Yeah I checked that. Right now I soldered them.
By using +/- 37 I change the 3k3 resistor to the 1k65R. Is n't it effective?

It is mounted to a heatsink, isn't it? The MJE340 ist there to track the temperature of the outputs and "correcting" the bias in one or the either way... Without contact it wouldn't work.

It's mounted to the heat sink. but the sound is still trouble until I put my finger on the buddy and another one to the +37.
 
I figured out something, When I put one my finger on positive rail and another finger on the collector of mje340 the sound get better very much but not good as should it be. I mark them in picture.
View attachment 824160

That's a good clue.

It looks like you have two small electrolytic capacitors for local bypass. Do you have any ceramic or film bypass capacitors on the board?

Try a 0.1 uF film cap of appropriate voltage rating directly across the power supply connections for the power transistors (collector to collector I guess in your circuit). Put it directly on the bottom of the board right underneath the transistors, from V+ to V-. Lose the long wire leads for the power transistors. Consider beefing up the electrolytic bypass capacitors and place them together in a star ground where the speaker ground and power supply ground all go directly to one point right by the capacitors.

Your physical layout is just all kinds of wrong and introduces inductance in all the wrong places! Even if the amplifier worked OK addressing these issues will clean up the sound. I do prototypes and sometimes they're not pretty but I still have to pay attention to stray inductance in a feedback circuit.
 

Anducer

Member
2011-03-21 3:44 pm
I think Fast Eddie is really getting to the heart of the issues:

Having those traces non-soldered on top that appear they should be connecting components, but really not, is adding some capacitance and inductive noise plus concerning questions why they exist.

Sorry i haven't analysed the full schematic and board (seems like a solid design at glance), but why all the "extra traces" on top?
 

Anducer

Member
2011-03-21 3:44 pm
Oh--and a thought that crosses my mind before i think of building anything audio (or a lot of circuit in general), is that i absolutely need some kind of oscilloscope to figure things out.

Also i have read (not having extreme experience as of late---mostly research first), that we should have everything prescribed as being on a heat sink, on a heat sink, before any kind of voltage applied and in test phase---you could degrade/destroy components easily even at low currents if not on heat sink---for many reasons (a lot to describe off top of head---i appreciate your effort and i hope to learn from your experience with this.
 
Bypass is your biggest problem.

Look at any commercial build. The power electrolytic capacitors are always together. You should do this too, with a large ground where the capacitors, power ground, and speaker ground all connect together. A smaller lead can come from this ground for signal (input) ground.

With your capacitors so far apart, you are creating a very large inductance in the ground where a very low inductance ground is needed. You are in effect begging for distortion and instability.

Ground layout makes or breaks audio circuits. Even cheap circuits work great with the correct ground layout, which by the way costs nothing. If you get the grounding right, you will have a quiet circuit with the lowest possible distortion.

Try the film cap too. I've done this to a lot of amplifiers and sometimes it cleans the sound up a lot, even when the amplifier was giving perfectly acceptable performance without it. It certainly helps with high frequency stability.
 
Before you go any further you have to solder ALL parts on BOTH sides or the circuit will not work. All of them. That includes those parts that you can't get underneath now like the electrolytic caps, power resistors, trimmers, etc. You might have to replace them with new ones with longer leads so you can solder the top side. There are no vias connecting the top to the bottom so you have to make that connection by soldering both sides, otherwise there are parts of the circuit that are not connected and it won't work.

Then mount the power transistors to the board, soldering both sides, and mount to a heat sink. Make sure you use some kind of thermal insulators on the transistors or you'll blow things up for certain. Only then should you power it back up again.

TJ
 

Anducer

Member
2011-03-21 3:44 pm
mshey, i was was only hinting at what chalky has stated to see if we would all see it the same including you.

Unless your pics that you show are not actually what you are testing on, this board needs to be redeveloped with traces on one side only---we should not encourage you to do anything else with this board and setup until you have a safer circuit and using best practices.
 

mshey904

Member
2020-03-10 7:35 pm
Try a 0.1 uF film cap of appropriate voltage rating directly across the power supply connections for the power transistors.
Consider beefing up the electrolytic bypass capacitors.

Bypass is your biggest problem.
Look at any commercial build. The power electrolytic capacitors are always together. You should do this too.

Try the film cap too.

I added 0.1 uF film cap, but the high distorted sound still exist.
Yeah, it's already connected with two 4700uF cap on +/- rails in the way that told.

Your physical layout is just all kinds of wrong and introduces inductance in all the wrong places! Even if the amplifier worked OK addressing these issues will clean up the sound. I do prototypes and sometimes they're not pretty but I still have to pay attention to stray inductance in a feedback circuit.

Having those traces non-soldered on top that appear they should be connecting components, but really not, is adding some capacitance and inductive noise plus concerning questions why they exist.
why all the "extra traces" on top?

Before you go any further you have to solder ALL parts on BOTH sides or the circuit will not work.

You are right guys I'll fix it before anything else. When I was making this circuit I didn't notice that I mirrored the layout twice. so I printed on another side.

Oh--and a thought that crosses my mind before i think of building anything audio (or a lot of circuit in general), is that i absolutely need some kind of oscilloscope to figure things out.

We should have everything prescribed as being on a heat sink.

Yes, I agree. But right now the price of a appropriate oscilloscope is a little high for me. I'll buy it later.
That's right, I'll take care of that. It was mounted to a heat sink but I had to use it for another STk amplifier.

You appear to have made a double sided pcb with the same print on both top and bottom - am I correct?

Yes, It happened because of a mistake when printing. So I printed another one at the second side. but I was wrong to didn't clean first side.

mshey, i was was only hinting at what chalky has stated to see if we would all see it the same including you.

this board needs to be redeveloped with traces on one side only.
[/QUOTE]

Yes Sure Anducer, I'll attach better picture. Do you mean that I take selfie with it?:D
I'll do it ASAP and report the result.
Right now I changed the heat sink temporary with this small one.

Thank you all for answering and helping.
IMG_2423.jpg
 

Anducer

Member
2011-03-21 3:44 pm
Yes, I agree. But right now the price of a appropriate oscilloscope is a little high for me. I'll buy it later.
That's right, I'll take care of that. It was mounted to a heat sink but I had to use it for another STk amplifier.

Totally understand, I've been in those shoes, but please consider looking up DIY O' scope interfaces to computer projects---you may be surprised that you can build an ok O' scope for use in audio just to trace your test (sine) signals---usually connect to sound card on computer for inexpensive ones.