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Old 22nd August 2003, 12:09 PM   #1
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Default Help! Problems with Bridged-Paralleled GainKlone.

I constructed the Bridged-Paralleled GainKlone using the attached circuit. I have built the inverting and non-inverting amps as individual amplifier units/stages. I am unable to parallel two opamp units, may it be the inverting amps or the non-inverting amps. However, bridging with one inverting and one non-inverting is ok. All individual units function nicely.

With paralleling of amps, current drawn is very high, as high as 3+ampere for one channel at no load. I suspect that there might a circulating current flowing between the paralleled amps.

In the attached circuit, feedback resistors are RC55 - 0.25W 0.1% the rest of them are 1% Metal Film but hand matched. The 0.5ohm 2W output resistors are actually two 10hm 1W 5% parallelled together, again hand matched.

The readings I am getting from the amps, as an individual module, are as follow,
@1kHz

Non-Inverting AMP1 AMP2
Peak-to peak voltage = 42 42 V
RMS = 14.89 14.92V
Average = -165 to -180, 17-35 mV

Inverting AMP1 AMP2
Peak-to peak voltage = 42 42 V
RMS = 14.89 14.92V
Average = -118 to -140, 33-53 mV

I wonder if the differences in these readings is acceptable. Which of these resdings should I pay more attention to? Has any one enounter this problems as well? How should I go about solving this problem?
Please Advice.

Thanks & Regards,
Yeo
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Old 22nd August 2003, 12:26 PM   #2
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I suggest that you read the application note AN-1192 and also thread here
http://www.diyaudio.com/forums/showt...884#post224884

The chips aren't very suitable to run DC-connected. Better to lower the offsets through a DC-servo or a cap in order to be able to reduce the 0.5 ohms resistors. They ruin the damping factor. They should be as small as possible.
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Old 22nd August 2003, 01:19 PM   #3
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Quote:
Originally posted by peranders

The chips aren't very suitable to run DC-connected. Better to lower the offsets through a DC-servo or a cap in order to be able to reduce the 0.5 ohms resistors. They ruin the damping factor. They should be as small as possible.

The million dollar question then is how Jeff Rowland does it on his power amps using LM3886s in series/parallel to get away with no servo on sight and 0.5 resistors at the output, as you can check on photos shown on this DIY Forum.


Carlos
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Old 22nd August 2003, 11:27 PM   #4
Previously: Kuei Yang Wang
 
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Konnichiwa,

Quote:
Originally posted by peranders
the 0.5 ohms resistors. They ruin the damping factor.
You mean the effective 0.5R output resistance ruins the damping factor more than the ten times as high voicecoil DCR? How. I mean in reality, not in irrelevant ivory tower measurements?

Other than that - yes, I think the chip's need individual Offset trim or DC blocking Capacitors per chip in the Inverting input link - 4u7 * 4 should do it. In addition however I'd also worry about the 2k5 Input impedance. The whole circuit is totally cockeyed like Trenco's Aether and should not work well, so hence it does not....

Excessive simplicity is as bad as excessive complexity. Make all things as simple as sensible, but no simpler.... AND NEVER simplify for reasons of cost, unless you are in the business of making money from HiFi gear....

Sayonara
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Old 23rd August 2003, 02:02 AM   #5
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Quote:
Originally posted by Kuei Yang Wang
Other than that - yes, I think the chip's need individual Offset trim or DC blocking Capacitors per chip in the Inverting input link - 4u7 * 4 should do it. In addition however I'd also worry about the 2k5 Input impedance. The whole circuit is totally cockeyed like Trenco's Aether and should not work well, so hence it does not....

Excessive simplicity is as bad as excessive complexity. Make all things as simple as sensible, but no simpler.... AND NEVER simplify for reasons of cost, unless you are in the business of making money from HiFi gear....

Sayonara [/B]
Input capacitor for each Opamp should solve the problem. I just realised the operations of two paralleled opamps are not isolated with one input capacitor shared by all opamps.

I may need to have a buffer stage before these amps as the input resistance is really too low. Have to see how it goes first.

Thanks
Yeo
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