This is not just another gainclone

[tiroth]

Perhaps you want INA134/7.

[lohk]

Can you tell me about the necessity of the resistor (and cap) from non-inverted input to ground in the inverted topology of the LM chip amp ?
I connected the positive input to ground directly and it sounds good; no noise, oscillation or other troubles.

Klaus

[Bricolo]

Originally posted by lohk
Can you tell me about the necessity of the resistor (and cap) from non-inverted input to ground in the inverted topology of the LM chip amp ?
I connected the positive input to ground directly and it sounds good; no noise, oscillation or other troubles.

Klaus

you can bypass them

[Rookie]

Hamish,

Which is the value of the capacitors you used in the PSU with OPA549? I know that OPA549 has less PSRR than LM3875 so does it need bigger capacitors than LM3875?

[ThorstenL]

Hi,

Why are you so in favor of line input transformers?

I am in favour of Transformers for Balanced Interfaces in general, inputs, outputs etc.

I see some benefits (much better CMRR in most cases) but also some detriments (much worse THD, especially at very low frequencies). Is trading THD for CMRR really worth it?

Well, first THD at low frequencies is not "MUCH WORSE" if you competently select the transformer, secondly, THD as such is one of the most meaningless Amplifier Specifications, together with Damping Factor. In the real world neither number has ANY bearing to the sound quality of a given amplifier.

However, the natural and passive bandwidth limiting behaviour of line input transformers can reliably protect the Amplifier input from ultrasonic noise that will cause signal dependent noisefloor modulation (a highly relevant factor to amplifier sonics not captured by ANY measurements) and provides a reliable and simle solution to a universal (balanced, unbalanced) input.

Of course, all this CAN be done electronically. To do it right requires several high performance op-amps, many passive components and still does not allow galvanic insulation, which is the only reliable way to kill noise loops.

As long as you are not bothered about interconnecting multiple pieces of equipment in the real world, with plenty of RF soup around from anything starting with VLFW from comms to nuke subs to your cellphone, with plenty of mainsborne noise and the like, as long as you connect the Amp in a lab to a AP One, instead of working it out under realistic conditions you will not understand the reason why you MUST use transformers if fidelity of the signal is desired.

Sayonara

[argo]

Originally posted by Bricolo
you can bypass them

I think he just did that?

Klaus

The extra resistor is a current bias-compensation resistor. It reduces the current bias by eliminating non-zero current at the inputs. Look also http://www.play-hookey.com/analog/ex..._inverter.html
About the cap I am not sure –should reduce power supply noise at non-inverting input?

Got my inverted clone up and running last night. I followed Peter Daniel’s minimal circuit –so only two resistors and input capacitor. Building this amp is well worth effort IMHO.
Sound is nice and polite not tiring, easy to listen, slightly romantic (as somebody else already mentioned). Details are all there, but not with extreme nicety to sidetrack your attention from overall music flow. Bass is a bit shy and low midrange could benefit by having more authority and darkness around. I think that it is because of small filter capacitors(1000uF) - will try 2200uF instead. Chips are cold as dead cats.
So far all is well.
The problems I encountered is PS hum (buzz) in output and that amp is extremely sensitive to mains disturbances (like from broken door bell or refrigerator activating)and RFI.
Circuit and PS grounds are all running into one small ground point (solder joint formed by all ground return wires). Inputs and outputs are isolated from chassis.
Could be that this too tight ground point causing the hum? Could bypass cap from ground to non-inverting input(as in Thorsten’s original schematic) solve the hum case. Could zobel at the output provide immunity to the outside radio noises?

Cheers,

Argo

[Philo]

I went out to my parents this weekend and hooked my inverted prototype(deadbug?) to my father's ESS AMT monitors. Not particularly a difficult load but tougher than anything I have and has better detail. The amp is almost dead quiet and sounded very nice. My father was very impressed once we compared it to all of his equipment (Dynacos, Nak TA4, and a few other various combinations). While it does have excellent range and staging, it does lack the real low end slam that larger discrete amps have, but the detail was there as well as the midrange warmth. We ran the amp hard for 2 hours and the 3"x4" x1/4" pieces of Al the ICs were screwed to never got warmer than 115degsF. I kept a .16ohm output resistor in to keep the amp stable with different loads, as well as a 2.0uF cap and 10k resistor on the input leg.

One question came up that maybe Kuei can answer... Once the filter caps are charged and I turn the amp off then back on I get a loud thump that I don't get went the amp has sat off for a while. I would have thought it would be the other way around because of the inrush from the filter caps charging up. I am not using the RC circuit to ground on the positive input, just shorted. Is this from not using any impedence on that leg and the charge on the caps just sitting on the input legs?

[protos]

Did you use it in a passive vol control set up ?

[Philo]

Yes, I forgot to add that. I am using a 100k linear pot before the input cap and resistor.

[leiade]

I have been away visiting a Swedish DIY forum (I did as you proposed mr Peter Daniel)for a while but I was put into the sinbin so I have to check this site for something new what is gainclone.