Help Needed to Sort out Problem in Crescendo ME Amplifier.

[sadik]

Dear Members,

Presently i am making Crescendo ME amplifier. I have Completed 1 Channel and Tested the same. Everything Works well so far. The Quiescent Current is 0.044 Volts. & The Audio is also Fine. Heat sink temperature is normal.

But there is one Problem for which in need Help from our Fellow Members on our Form.I have attached Schematic Diagram of same or Can be Downloaded form my Google Drive by clicking on the link given at end of this article.

Before describing my Problem i would like to share the equivalent parts used.
1. C3 in Amplifier Board is 180n where as i have used is 150n.
2. R4 & R5 Resister in Amplifier Board is 1.8m where as I have used 1.5 M +330 K in Series which makes 1.83M.
3. In Protection Board Original Pre Amp IC is OP77 where as i am using OP07.

Problem is described below :
1. When I am Measuring Current in Speaker Output (tp3 as in schematic) i am getting 0.4 Volts. This Should be 0 Volt or 0.1 Volt.
2. When i attach the Amplifier Board with Protection Board and raise the Volume, the Protection Turns On and trips the Sound Output. (Without Protection Board I can increase the volume as much as i like and there is no abnormal behavior with Amplifier Board Except the Voltage at tp3 i get is 0.4 Volts)

Can anyone please check the schematic of same and suggest what changes to be made to Get 0 V at output. I have read on DIYaudio for such Problem but that didn't help me.

Also i would request to you that please mention Part No. ( R1, R2 etc) as mentioned in schematic if any changes to be made. I Apologize for same because by Profession I am Mechanical Engineer & I cannot understand the Complex Theories in Electronics, So If i ask some silly questions which I may not be able to understand regarding your suggestions/changes Please do not Mind.

Link : https://drive.google.com/folderview?id=0B2a9DN9GbmtBOXJvNTlGUHppRXM&usp=sharing

Sadik Bhatkar.

 

[shaan]

Hi Sadik.

Where are you connecting the "Bias" node?
(wrongly named though. it controls the offset, not bias which is controlled by the 1K pot near the MJE340)

 

[sadik]

The Bais node is connected to the Bais Point given in Protection Board.

The Wiring Diagram is also shown in last page of pdf Document on Google Drive.

 

[shaan]

Are you sure the protection is working okay and no wiring mistakes? As in the circuit, and if the opamp is not damaged the offset should be like 1mV!

 

[sadik]

Are you sure the protection is working okay and no wiring mistakes? As in the circuit, and if the opamp is not damaged the offset should be like 1mV!

Dear Shaan,

If i have to just check the amplifier board can you please let me know where should i connect the Bais Point given on Amp Board.

 

[Aniket]

Hello Sadik,

try procuring 1M8 for R4 and R5. 0.4V DC on the output is pretty high DC offset.
increase C3, C4 in the protection circuit to 220uF, and check if the protection still trips on high volume.
i would have rather gone with a simplistic protection circuit and played around with R2(although it would change the input impedance) to check the DC offset instead of using a DC servo. keep it simple.

Regards,
Aniket

 

[sadik]

Hello Sadik,

try procuring 1M8 for R4 and R5. 0.4V DC on the output is pretty high DC offset.
increase C3, C4 in the protection circuit to 220uF, and check if the protection still trips on high volume.
i would have rather gone with a simplistic protection circuit and played around with R2(although it would change the input impedance) to check the DC offset instead of using a DC servo. keep it simple.

Regards,
Aniket

Thank you Aniket for your suggestion i will try tomorrow and see... Getting 1.8M here locally is not possible... i searched a lot for same. But i will try your other suggestions.

 

[aks07]

Interesting circuit. I think this was published in EFY mag many years ago.

Looking at the topology it looks like a challenging project. Much will depend on PCB layout, quality of components etc to achieve stability without turning it into an oscillator.

From your description it seems like DC offset situation. It will never be 0 but a brand new amplifier should settle down to a value no more than 5 millivolt after warm up. Between 20 millivolt to 50 millivolt it may start effecting your output waveform and around 150 ~ 200 millivolt the DC protection will start to kick in. This is happening in your case so your protection board seems to be working as intended.

- Firstly check that all the transistors are matched closely to their reciprocating number (e.i. T5 with T6, etc).

- Check if R34 and R35 are exactly matched. It doesn't matter if the value is.25 or .20 ohm but both should have the exact same value. Try to use non inductive resistors.

- Check if the D3 and D4 are perfectly matched with each other at 3.9v.

- Check the quality of transistors BC546/556. Nowadays most such devices are very low quality china maal. Consider putting a better replacement. I think something like mps8099/mps8599*should work fine.

Optionally you may consider that following which may not really help with DC offset issue but could help if the amp is oscillating at some stage.

- A small cap across the pot P1
- Bypassing C6, C13, C7 and C15 with a 100n film capacitor right at the leg.
- Experimenting with the values of the output RC network. You can even shift the network to binding posts.

Honestly, I have never built or troublshooted this type of schematic. So that all the above is FWIW. I am just commenting by cursorily looking at the circuit and trying to figure out what could be causing the problems which you have described. But the above steps may fix the issues assuming you have built it exactly as per plan using good components.

I am still thinking what was the key design goal in this schematic? Maximise open loop gain?

 

[sadik]

Today i assembled a new Protection Board.. And started testing my amp ... I got all the values exact which are written.. Offset is 0.44 Volts and DC output is between 0 & 0.15 Milli volts. (Keeps on fluctuating), but most of the time it is 0. But now a new Problem had rise. The Output from Protection Card is not Working. May be i may have damaged the Relays while Removing form the OLD PCB. Tomorrow i will get new relays and check back again.

@ aks07 ... I Donno what was the key design goal of this schematic, But it seems to be very good amp by looking at the measured results which is shown on second page of the pdf Document. I have heard many DIY'ers Praising this amp who have built it in past. For me I started this Project because it was very well documented, Didnt know i can face such issues.

 

[diesel engine]

400mV is a tad bit high. are the low signal trannies _(T1-T6) matched pairs? try using hfe matched pairs. use matched pairs in both channels if possible. this may help dealing with the offset problem.

 

[sadik]

I rechecked the Protection kit and Found the Problem. I was so stupid, Instead if using BC546B i used BC556. Yesterday i changed the transistors and putted the correct value and tested the Amp. The Results were Success. At last i succeeded in building this amp.

During this built i got to know many things specially the benefits of using hfe matched transistors.Thank you all my friends for your suggestions. And Specially i would like to thank you Shaan for your Support and guiding me to sort out my Problem.

@diesel engine you are absolutely right, it is very much beneficial to use hfe matched pairs. Next time while taking on some new Project, this Experience will help me a lot.

Thank you friends once again for your suggestions.

 

[sadik]

Hi Friends..

I have completed assembly and testing of both the channels.. The results are awesome.. i don't figure out if any frequency is missing out there while testing.
A Quick view on specifications and measurements of this amp which was published by "Elektor"

Below Image is the DC Milli volts Reading at Output

Next Image is Both the Channels in Action... I Mean Testing..

Now the Task Begins for designing an Enclosure for my Amp...

 

[shaan]

Now the Task Begins for designing an Enclosure for my Amp...

Congrats. Looking forward!

 

[diesel engine]

Hi, it is nice to know that you sorted it out. congrats and more successful DIYing!

 

[sadik]

Hi, it is nice to know that you sorted it out. congrats and more successful DIYing!

Thankx Diesel ... Its Now time to Design and Built the Enclosure for same. The Enclosure is on the way..