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Old 26th February 2009, 06:51 PM   #21
sendler is offline sendler  United States
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Default Trim pots

Quote:
Originally posted by CJ900RR



Except that the Jeff Rowland design is not inverted
.
If the Rowland is not inverted what do you suppose the trim pots on each output are for?
.
As to the balanced conversion of the input, shine 7 eventually did change to transformers as they sounded better.
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Old 26th February 2009, 07:33 PM   #22
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But it's not close enough to get similar performance. At least Rowland doesn't using so high value resistors, likewise I can't observe any big supply bypassing or decoupling caps near the chips in his Concentra/Concentra II Integrated Amplifier. A 10K gain setting resistor is clearly visible. At the right side there are only perhaps 100nF bypass caps. A feedback resistor connected directly to Ground pin (to minimize trace resistance and inductance, as well as DC offset I guess), so signal ground is omitted. However what is it a strange R29 resistor?
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Old 26th February 2009, 07:56 PM   #23
CJ900RR is offline CJ900RR  Sweden
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Default Re: Trim pots

Quote:
Originally posted by sendler

.
If the Rowland is not inverted what do you suppose the trim pots on each output are for?
.
As to the balanced conversion of the input, shine 7 eventually did change to transformers as they sounded better.
The pot is not connected to the output, it's connected to one of the inputs.

A dc-servo can be applied to either positive or negative input. The pot is a simplified, manual, dc-servo so why shouldn't it work on a non-inverted amplifier?

And what is the resistor connected to pin 10 (R111, R104...) on the LM3886 at the picture of the Model 10 amplifier? Shouldn't it be connected to the pot if it was a inverted design?

I have not seen the schematic for Model 10 so correct me if I'm wrong. To me it looks like a non-inverted design...

Edit: regarding the change to from DRV134 to a transformer; offcourse it must have sounded better, the transformer is made for the job (depending of how you connect it offcourse), the DRV134 isn't. You don't use a balanced line-driver do bridge a amplifier, you use a bridging adaptor to bridge amplifiers!
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Old 26th February 2009, 08:53 PM   #24
sendler is offline sendler  United States
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Default dc offset trim circuit for noninverted?

Quote:
Originally posted by CJ900RR


The pot is not connected to the output, it's connected to one of the inputs.
.
I meant to say near each output device.
.
Can you show a sketch of how they might be using the trim pot to trim dc offset in a non inverted amp? I can't see the traces but can see that there isn't enough showing to make a servo.
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Old 26th February 2009, 09:53 PM   #25
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Default Re: Re: Trim pots

Quote:
Originally posted by CJ900RR
regarding the change to from DRV134 to a transformer; offcourse it must have sounded better, the transformer is made for the job (depending of how you connect it offcourse), the DRV134 isn't. You don't use a balanced line-driver do bridge a amplifier, you use a bridging adaptor to bridge amplifiers!
Can you back this up with a techincal argument of some kind? TIA
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Old 26th February 2009, 10:16 PM   #26
CJ900RR is offline CJ900RR  Sweden
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Default Re: Re: Re: Trim pots

Quote:
Originally posted by sendler

.
I meant to say near each output device.
.
Can you show a sketch of how they might be using the trim pot to trim dc offset in a non inverted amp? I can't see the traces but can see that there isn't enough showing to make a servo.
Well I might have to back down on this one. I was thinking about a design that works on ordinary OP-amps but now I'm not so sure it works on LM3886. Circuit below, not tested!

Click the image to open in full size.

Quote:
Originally posted by leadbelly


Can you back this up with a techincal argument of some kind? TIA
DRV134 datasheet says:

The DRV134 (and DRV135 in SO-8 package) converts single-ended, ground-referenced input to a floating differential output with +6dB gain (G = 2).

The BPA300 (and almost every other LM3xxx-based amplifiers) have a unbalanced, single-ended input. To bridge this amplifiers you need to convert one unbalanced signal to one unbalanced, non-inverted and one unbalanced, inverted signal. The signals need to be unbalanced, referenced to ground, and not a floating differential signal.

The DRV134 is not suitable for the job and there can be issues when using it for that. The job can easily been done with a dual OP-amp and some resistors instead.
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Old 26th February 2009, 10:36 PM   #27
sendler is offline sendler  United States
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Default Re: Re: Re: Re: Trim pots

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Originally posted by CJ900RR


Well I might have to back down on this one. I was thinking about a design that works on ordinary OP-amps but now I'm not so sure it works on LM3886. Circuit below, not tested!

Click the image to open in full size.

That could work. Thanks for getting back with that so fast. That would explain the seemingly extra resistor in the photo. I have 8 3875s arriving tomorrow in order to try a bridged/ parallel. I was going to build them inverted to get rid of the de-coupling cap but may try that dc offset circuit with non-inverted instead to keep a reasonable input impedance for use with my passive stepped attenuators.
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Old 26th February 2009, 11:02 PM   #28
CJ900RR is offline CJ900RR  Sweden
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Default Re: Re: Re: Re: Re: Trim pots

Quote:
Originally posted by sendler



That could work. Thanks for getting back with that so fast. That would explain the seemingly extra resistor in the photo. I have 8 3875s arriving tomorrow in order to try a bridged/ parallel. I was going to build them inverted to get rid of the de-coupling cap but may try that dc offset circuit with non-inverted instead to keep a reasonable input impedance for use with my passive stepped attenuators.

No problem, but notice that I have not tried this myself! I don't guarantee that it works with LM38xx!

I also thought that the extra resistor was strange and therefore I think that they might use something like this above.

Note, R5 might not be needed and there is a more simplified construction: remove R4 and R5 and connect R3 directly to the center-tap of the pot....
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Old 26th February 2009, 11:02 PM   #29
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Default Re: Re: Re: Re: Trim pots

Quote:
Originally posted by CJ900RR
The BPA300 (and almost every other LM3xxx-based amplifiers) have a unbalanced, single-ended input. To bridge this amplifiers you need to convert one unbalanced signal to one unbalanced, non-inverted and one unbalanced, inverted signal. The signals need to be unbalanced, referenced to ground, and not a floating differential signal.
1) The signals do not need to be unbalanced, where did you get that from? If anything, being balanced makes a better performing amp.

2) What does the fact that it is floating have to do with anything? If it was a problem, add some resistors to reference it to ground.
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Old 26th February 2009, 11:23 PM   #30
CJ900RR is offline CJ900RR  Sweden
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Default Re: Re: Re: Re: Re: Trim pots

Quote:
Originally posted by leadbelly


1) The signals do not need to be unbalanced, where did you get that from? If anything, being balanced makes a better performing amp.

2) What does the fact that it is floating have to do with anything? If it was a problem, add some resistors to reference it to ground.
1) No it don't need to be balanced, but my statement was regarding the design at Shine7, witch does not have a balanced input. Indeed balanced signal makes better performance, if the design is constructed as balanced! The BPA300 is constructed for single, unbalanced, earth-referenced inputs, not differental, floating inputs!

2) The fact? Don't you want two boards that "see" the same signal, except that one is inverted? Yes you do. There is differences in the signal from the DRV134. Have you measured the frequency-signal on each board bridged with a DRV134, and how does it look? Sure, use resistors to ground but how about the high and undefined offset?

It could work pretty well with AC-coupled amps though...
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