SOAP MKIII (Simple OpAmp Pre) With onboard PSU

[Nuuk]

Andrew, the circuit I posted was built as a pre between a Monica2 DAC and a Charlize amp. I did quite a lot of experimenting before I settled on the gain etc. And CarlosFM suggested the other tweaks.

As far as I know, it is still working fine for the guy I built it for and he was very happy with the sound quality (as was I). Neither OPA134 chip seemed to get hot.

What is the minimum load impedance and maximum cable capacitance that the ClassA output stage can feed?

That I honestly can't tell you but perhaps I should add that it was running on +/-12 volt rails (SMPS).

(Full story here )

[peranders]

Originally posted by Nuuk
Here are some other tweaks that you may like to try too.



Yes, it's the single version of the 2134 but the same applies.

I used 100uF/35v Panasonic for audio types on each power supply pin. The DC blocking was taken care of by caps in the DAC and the power amp.

Nuuk, this class A thing should be applied with care. Not every opamp will benefit from this and in your case seems 10 mA crazy. The opamp will run pretty hot and the gain is little.

[zei]

yes C5 & C6 for decoupling. but do they need to be snubbed or electro//film or +ve to -ve or are ceramic OK?

I can,t see why ceramics should,nt work however i was thinking normal plastic types here. The 3D is showing the wrong kind..

On the prototype Micas are used...


/JZ

[peranders]

Ceramics are very good if you do it right.

[zei]

All in all..

The SOAP sounds very good, is cheap and easy to build.

The onboard psu makes it very simple to put in a chassi etc..

I,m very pleased with the result.


I might even have a few boards "pro" made .


/JZ

[Nuuk]

Andrew and Peranders,

I appreciate the concern and you guys are much more knowledgeable than I am.

I wouldn't use that set-up on any other opamp without consultation, but as I said already, it appears to have worked in the circuit shown without problem for a couple of years now without the OPA134's failing.

In fact, it sounded so good, if the opamps did need replacing each year, it would still be worth consideration IMHO.

[peranders]

The temperature is probably not an issue but 10 mA + equally much at positive signals plus the load will result in 20 mA at least and this is rather much for the opamp if you want ultra low distortion. Try to do a blind test with and without this circuit. Can you pick the difference if you use a good opamp.

[AndrewT]

Hi Nuuk,
12V gives just 8mA of bias and that adds about 90mW of dissipation to a 134.
Survivable in the longer term.

But 15V and 2134 and suddenly the extra dissipation is up around 280mW. That gets the opamp into a completely different temperature regime.

[Nuuk]

Thanks Andrew,

I have now added a note attached to that circuit that it should not be used with a higher voltage supply.

[peranders]

You can use a class A circuit at higher voltages but you'll have to choose the current with taste.