Replacing TL074 opamp in Soundstream CA 3.0

[JLH]

I performed a search and didn't find anything specific to the Soundstream amps. I've been busy repairing a CA 6.0 (works now!) and started on two CA 3.0 amps. Since the 3.0's will be driving mids and HLCDs I'd like to upgrade a little bit as I repair them. I'm not after more power, just SQ. With the opamps being TL074 this would seem like a very obvious place to start. There are a lot of pin compatable opamps out there that out perform the TL074. I have some Burr-Brown OPA4134 on hand that I'm going to try. Good move, bad move?

I'm also replacing C206 with an Elna audio grade cap. I going to try to squeeze in an audio grade polypropylene cap for C201. I know the Picasso used poly caps and it was one of the best sounding amps I've heard. Running an audio signal through an electrolytic cap just makes me cringe.

Lastly, looking at the schematic, I can't quite figure out what they are trying to do in the first opamp stage. Forgive me its been 20 years since I messed with opamp circuits. However, it looks like they have a integrator circuit set to low pass at 90KHz. This puts the high frequencies -1.63dB at 20KHz? The thing that bothers me is it looks like the voltage gain of the first stage is 13.4 (+22.5dB). Isn't that pretty high? the second stage is clearly a high pass rumble filter set to 18Hz with a voltage gain of about 1.9 (+5.6dB).

The old Class 50 II, D100 II, and D200 did not have these opamp stages. Why are they in the CA 3.0 and Ref 300 amps? Thanks guys.

Rgs, JLH

[Perry Babin]

According to the attached graph, it appears that the signal is only down by 0.12dB at 20k.

[JLH]

Originally Posted by Perry Babin

According to the attached graph, it appears that the signal is only down by 0.12dB at 20k.

Interesting. Oh well, I said I was out of practice. That's still a lot of gain that in my opinion doesn't need to be there. Seems you could quiet the noise level of the amp if the gain was reduced.

Rgs, JLH

[Perry Babin]

They may have wanted to get the signal level high enough to swamp out noise from later stages. If the gain was achieved in later stages, that would amplify noise from earlier stages/circuits.

If you find that you have sufficient input signal to easily drive the amp to full power with the gain at the maximum position, you could reduce the gain in this stage to lower the noise floor. I'd prefer to see you do this instead of replacing op-amps.

[JLH]

To the lower the gain I'm thinking of doing the following: Change R202 to 24.9K, change C202 to 75pF, and change R203 to 49.9K. This should drop the voltage gain of the first stage from 13.4 down to 1.5 while keeping close to the 90KHz low pass.

Rgs, JLH

[JLH]

Originally Posted by JLH

To the lower the gain I'm thinking of doing the following: Change R202 to 24.9K, change C202 to 75pF, and change R203 to 49.9K. This should drop the voltage gain of the first stage from 13.4 down to 1.5 while keeping close to the 90KHz low pass.

Rgs, JLH

I forgot to mention that I use an Audio Control EQX in my system which can output up to 9.5Vrms, so I have lots of voltage headroom to spare.

[1moreamp]

Originally Posted by JLH

I forgot to mention that I use an Audio Control EQX in my system which can output up to 9.5Vrms, so I have lots of voltage headroom to spare.

Most Op-Amp filters use Op-Amp output buffer stages to prevent the filters characteristics from being interfered with by impedance loading related issues that sometimes occur by the next stage. Generally on most car amps you will see a input buffer op-amp stage, then a filter/variable gain stage of some sort, and then a output buffer stage follow up. It may look redundant but you do see it on most every op-amp design that uses any sort of filter or gain control circuit...

If your down on electrolytic's then try Film type caps instead, lots of highly rated SQ amps do this.
Hope this helps some..

[azvrt]

A person working for one of the better car amplifier manufacturers recently told me he had very good results on Soundstreams with these simple modifications:

"The SQ upgrades consist of replacing the op amps with higher spec parts (LME49740) and fitting uprated main power supply caps.

On most amps there are 1 or 2 pairs of 1000uF caps, which are way too small. I'm fitting 3300uF 50v radial caps in place of one pair of 1000uF caps and laying them down so they will fit.

The can size is 18 x 35, and there's 19-20mm between the board and back cover so they fit comfortably.

This increases the capacitance from 2000uF to 4300uF per rail, still a bit light for my liking but more than twice as much as standard.

The extra capacity improves power output, and will improve dynamics as the amp will be able to put out more peak current."

But some people believe doing upgrades is quite useless.
I can't comment myself as I have never tried it (yet).

[JLH]

Still slowly working my way through the first 3.0 still. One thing about these amps that never occured to me is the rectification scheme appears to be a pair of half wave voltage doublers. Why? Wouldn't a standard center tapped full wave bridge work as good if not better and be less complicated?

[Perry Babin]

The doubler rectifiers are used to form a bridge rectifier. The transformer drives the center leg. The positive and negative outputs are on the outer legs of the doublers.