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Old 8th December 2012, 02:32 AM   #1071
1audio is offline 1audio  United States
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The changes may not show on the meter at this stage. The input noise and bandwidth could be the limiter. Even with lower noise input devices the input attenuator becomes the limit for noise. Using the FFT help see into the noise a lot.
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Old 8th December 2012, 04:54 AM   #1072
RNMarsh is offline RNMarsh  United States
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The meter as it is isnt useful in resolving thd for DUT under -100dB.... the bottom 10% of the scale.... but even so, the FFT will get used.... just needed a good low thd flexible signal source. And, then there are the other sources.. like Victors or the one SW just build if DUT is really, really, really low. In either case, I am very curious about the prototype system davada is using.
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Last edited by RNMarsh; 8th December 2012 at 04:58 AM.
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Old 8th December 2012, 06:56 AM   #1073
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
The meter as it is isnt useful in resolving thd for DUT under -100dB.... the bottom 10% of the scale.... but even so, the FFT will get used.... just needed a good low thd flexible signal source. And, then there are the other sources.. like Victors or the one SW just build if DUT is really, really, really low. In either case, I am very curious about the prototype system davada is using.
Thx-RNMarsh
Hi Rick,

I loaded more LT1468 and high frequency noise and parasitic is out of control.
Worse than ever. Each stage just amplifies it more and it keeps getting worse as we go along.

HP compensated each stage narrowing the bandwidth on the 2625's and this is how they made it work. I will have to deal with each stage to tame it and in the end I might just end up with what we have in the first place.

I think I will just focus on the input, buffer and notch filter since this is all fairly well behaved and forget about the rest.

I think Dick has it right with his Twin T running on batteries, a precision pot for an attenuator and FFT.

Lets face it the 339a is a boat anchor. It works well as a level meter and that's about it.

I'm getting good results with my original idea of just using it for a front end.

And please call me David.

Cheers,
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Old 8th December 2012, 08:27 AM   #1074
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FRont end use is fine and the best part of the product. I think you've got a good handle on that.
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Old 8th December 2012, 05:29 PM   #1075
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FRont end use is fine and the best part of the product. I think you've got a good handle on that.
Maybe those opamps are not the best to use.... we dont need BW, just low noise and lowest THD. Like maybe the AD711/12/13 would be better in some places or others; more stable and less prone to oscillation.
Its clear from the pcb and wiring layout, that it cant be used for widebandwidth circuits.... long and convoluted wiring and all. <100KHz layout. -RNMarsh

Last edited by RNMarsh; 8th December 2012 at 05:52 PM. Reason: use more stable OPA -
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Old 8th December 2012, 06:20 PM   #1076
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Maybe those opamps are not the best to use.... we dont need BW, just low noise and lowest THD. Like maybe the AD711/12/13 would be better in some places or others; more stable and less prone to oscillation.
Its clear from the pcb and wiring layout, that it cant be used for widebandwidth circuits.... long and convoluted wiring and all. <100KHz layout. -RNMarsh
Some others might be better -- OPA2604, LME49990, LM4562 etc.
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Old 8th December 2012, 08:20 PM   #1077
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
Maybe those opamps are not the best to use.... we dont need BW, just low noise and lowest THD. Like maybe the AD711/12/13 would be better in some places or others; more stable and less prone to oscillation.
Its clear from the pcb and wiring layout, that it cant be used for widebandwidth circuits.... long and convoluted wiring and all. <100KHz layout. -RNMarsh

Hi Rick,

I have the input amplifier tamed for wide band use. If we are to do a THD20 at 20KHz then we need a bandwidth of at leas 400KHz. One thing I noticed is there is not much for input LP filtering. I going to work on this. The LT1468 is particularly low in distortion at audio frequencies. It just needs a little TLC and there are very stable once we get it right.
Even with lower GBP op amps there will still be RF problem to deal with. Any op amp with a GBP 10MHz or greater will go into the 100's of MHz at gains below unity so there is no escaping this. The problem is with the long signal chain.

There is no inter stage LP filtering and so the RF gets passed from stage to stage. I'm going to work on this as well.

I found my unit is not wired the same as any of the manuals I have and this might explain why my unit behaves a bit differently from others.

I by passed the buffer amp at the input which is routed totally differently from the schematic. The noise dropped about half a order magnitude on the meter and with the input shorted the meter goes darn near zero. Of course without signal the auto set level shuts down. Bypassing the the buffer amp does raise the 2nd H a bit because of the added loading on the input amp. I'm going to try a different op amp here maybe a LME49710. Low noise high drive current and low distortion. After all it's just a buffer.

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Old 8th December 2012, 08:27 PM   #1078
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Some others might be better -- OPA2604, LME49990, LM4562 etc.

With my SVOscillator I found the LT1468 out perform the LME, LM series op amps. Haven't tried the OPA2604. The OPA1641 and LT1468 are far better when it come to covering a large range of impedance on the noise curve. What we want is a very low current noise figure.
if it wasn't for the high impedance I would use the OPA1611.

And yes the 339a routing is terrible.

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Old 8th December 2012, 11:36 PM   #1079
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I'm pretty much at the point where there are three opamps (or their duals and quads) for me for AC use -- LME49710, LT1468, OPA1641. Everything else seems to be for GP or utility work...
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Old 8th December 2012, 11:54 PM   #1080
RNMarsh is offline RNMarsh  United States
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The 339A is beginning to see the effects of the THD of the opamps used in itself and also the parts and even contacts of the switches. All issues which an audio amp builder would also come to address at these levels of harmonics. The performance will be limited by the parts we use. I am installing sockets everywhere to try different opamps in different places.... will be much easier until the parts are settled as to which are best ones and stable. Thx-RNMarsh
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