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Help on perfboard A47

post #1 of 11
10/5/11 at 6:28am
Thread Starter 

Tinkering with a A47 build with leftover parts from other projects. Ran into some problems. While powering it with a 9v battery, I get the same readout on V+, V- and ground. I've also tried a wallwart PSU with the same results. The sound is dirty and only comes out the right channel. The layout it the same thats been seen here, Ill repost it, along with pictures. Any help is more than welcome!

 

a47_front.jpg

 

 

a47_back.jpg

 

a47_schem.jpg

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post #2 of 11
10/5/11 at 6:47am

First step, take the op-amps out of their sockets and check the power supply voltages.

Make sure the voltages appear at the proper pins in the sockets. Once you have the

power sorted out, try the op-amps again. Check the voltages with the op-amps installed.

 

What is the voltage reading from the battery?

What is the voltage from - to the "common" on the TLE

What is the voltage from the - to the output of the BUF634?

Don't forget to try a different battery as well...

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post #3 of 11
10/5/11 at 9:38am
Thread Starter 
Quote:
Originally Posted by Avro_Arrow View Post

First step, take the op-amps out of their sockets and check the power supply voltages.

Make sure the voltages appear at the proper pins in the sockets. Once you have the

power sorted out, try the op-amps again. Check the voltages with the op-amps installed.

 

What is the voltage reading from the battery?

What is the voltage from - to the "common" on the TLE

What is the voltage from the - to the output of the BUF634?

Don't forget to try a different battery as well...

 

Had the V- 2 socket on the wrong pin, corrected that. I measured the sockets with the opamps out, I get 8v on pins 4 and 8 on V+ side and pins 1 and 5 on the V- side. The odd thing is that I have 4.4v on all other pins on the socket, is this correct?

 

The other voltages are:

 

Battery: 8,95v
- to common: 4,4v
- to output: 4,4v

 

Comments?

 

 

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post #4 of 11
10/5/11 at 11:17am

Yes, the other pins should show 4.4 volts with the op-amps removed.

Try with the op-amps again and see if it works better with your repairs.

If one channel still does not work, swap the op-amps around and see

if the problem follows the op-amp. If it does, then that op-amp is bad.


Edited by Avro_Arrow - 10/5/11 at 11:20am
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post #5 of 11
10/5/11 at 12:04pm
Thread Starter 

Allright, changed the OPA2132PA:s for OPA2132P:s. Good sound on left channel - no sound at all in the right. Tried reflowing the output caps on the right side but no help there. 

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post #6 of 11
10/5/11 at 12:47pm

The only spot I can find that look questionable is here:

 

a47_back.jpeg

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post #7 of 11
10/5/11 at 10:36pm
Thread Starter 

Got the amp working. Probably was a bad solder on the output cap, on the spot you pointed out. Fit really snugly in a Hammond 1593P case, had to cut the top of the BUF634. 

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post #8 of 11
10/6/11 at 5:12am

Glad to hear you were successful!

 

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post #9 of 11
10/6/11 at 8:43am
Thread Starter 

The thanks go to you Avro_Arrow, the best thing about Head-Fi is that you can get almost instant support on these DIY projects, a very important factor so you don't stress yourself out with your doings. I would be interested in knowing the importance of the BUF634 in this amp, as this is my third amp this far, I haven't run into a circuit using this chip before. 

 

Also, on opamps, I read a few threads were some people recommend mixing opamps and some are totally against it, any takes on this? Is it safe to have say, OPA2111 on first place and OPA2132:s in the other? The sound is also a bit thin currently, some lack on the bass side. I used the recommended values for electrolytic and output caps, testing now on Sennheiser HD600:s


Edited by lehtinel - 10/6/11 at 8:45am
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post #10 of 11
10/6/11 at 9:50am

When you are saying "output capacitor", you are really

referring to the Input capacitors. You could increase the value

of these caps to 1uF or even 2.2uF. This could help firm

up you low frequency response.

 

The function of the BUF634 in your A47 is easy to explain.

In "CMoy" type amps, a resistor divider or a TLE2426 are

used to create a "midpoint" voltage between the battery

positive and negative voltages. This is also called a "virtual ground".

This type of divider would not be able to provide enough current for

the needs of an A47 amp. The BUF634 is used to increases the

current available from the virtual ground

.

For an A47, I would not recommend mixing op amp types.

One of the two op amps is wired to serve as the left channel

and the other one for the right. Mixing type would have obvious

side effects...

 

You can increase the performance of the A47 by wrapping the BUF634

in a feedback loop. This will lower the output resistance of the BUF634.

But that would mean re-building what you have now. Maybe something

to do for the next version...

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post #11 of 11
10/7/11 at 4:02am
Thread Starter 

True, mixed up the output / input cap, clogged up brain with flu symptoms. Thanks for the theory lesson, will see if I change the input caps, what I've got is 0.1uF Wimas currently, but might order some 1uF or higher capacitance when I need something else.

 

Would like to try the OPA2111 opamps on this amp also. Will see.

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