Crown PSA-2 servicing and upgrading

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V9977

Well-known member
Joined
Nov 22, 2004
Messages
170
Location
Athens, Greece
We have here a Crown PSA-2 stereo amp for monitoring purposes which I am
currently renovating. It employs TL074 and a single TL071 on the main board
driving the output power transistors. Would it be possible to replace with OPA4134PA
or OPA4132PA without upsetting some other super-tuned parameter in this circuit?
Schematic can be found here: http://www.crownaudio.com/pdf/legacy/psa2schematics.pdf

The single opamp on the left is TL071 while the ones in the middle are TL074 quad package.
Also, what do people think of this amp?  ;)

Thank you.
 
Hi,


  They were very expensive, well built, but I never liked them. When I worked for Quested, we had a couple of customers that used them. They were really harsh on the mid and top, flabby and uncontrolled on the bottom end, and those customers used to pop midranges and tweeters like they were going out of fashion. At the same time, I used to come across quite a few people that swore by them on their NS10s, so who knows, perhaps they are good on fullrange . . . . I am sure you would have a fantastic headphone amp for multiple setups.

  they are pretty reliable iirc


  sorry to be negative . . . . .


      ANdyP
 
I would not advise replacing the opamps, since, as you mentioned, the PSA is super-tuned. TLO series have more than decent performance, even by today's standards.
The PS200 and 400 used opamps (IIRC UA739) that were hard to source, and all tentatives to replace them were met with disappointment, instability and fried output transistors.
 
@Bouncy: That would be one massive headphone amp! Thank you for the input, I am interested
in the negative just as much as the positive like all of us around here.

@abbey road: Thanks for reply. I am concerned that the single TL071 on the left might be sensitive to changing,
even though the OPA134/132 is supposedly a direct replacement, no? I must say I've had massive improvements
with this move in the past. UA739 are not used in this model (PSA-200) at least on our unit and in the service manual.
 
I will add another vote, to lower your expectations for sonic improvement from swapping out opamps. While the TL07x is suitably fast, it is not noted for robust drive capability, but inside a large power amp, the heavy lifting is being done by additional discrete transistors, which will likely dominate your linearity performance.

Unless you are very familiar with power amps I would advise caution, since you can do some serious damage if you make it unstable. If you were familiar, you probably wouldn't be asking this question. 

I am not a huge fan of crown power amp design, but they are surely one of the small handful of major players, and competent. If they used a TL07x they are already using relatively high performance parts for that application.

As a circuit designer I object to the general sense that isolated component upgrades can make dramatic differences in circuit performance. An audio power amplifier has many moving parts. A slow crappy opamp could hurt performance, but a perfect one, is not likely to make a huge difference for the better, because of all the other links in that signal chain that are doing heavy work.

JR
 
I am familliar enough with power amps which is why I am asking about substitution in this case.
Thank you for your thoughts and experience.
 
Quick update and a couple of questions:

Part 1: To align the compressor at the balanced input stage, a 10V p-p square wave with a 5V DC offset is needed.
As I do not have a generator capable of providing this at 9Hz, I have drawn up a simple circuit which can
shape a sine input (from DAC) into square with the desirable offset. Note Germanium cliping diodes.

Any thoughts?

SquarewaveGenerator.jpg







Part 2: I am geting the classic 'CLICK' when something is plugged into an adjacent socket (eg. soldering iron).

Thank you.
This is varying in amplitude but I also got a very loud 'POP' when my darling switched-on the kitchen ventilator.
Other equipment is just fine in the mains setup, and it was doing it at the studio as well.
It has now been recaped throughout appart from the 10000uF 100V reservoir electrolytics. Could these be the issue?

Have tried connecting a couple of 0.33uF MKP in parallel to bring the impedance down at HF but no difference whatsoever.
 

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