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Old 4th January 2012, 09:10 PM   #11
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I'd suggest leaving the op-amps. Any change in sound quality is likely to be imaginary and unless someone has tried the op-amps that you use as replacements, it may create more problems.

The balanced/normal switch rarely makes a difference in level. Can you move it and cause the level to go up and down?

If not, the problem was a dirty contact in the high/low level switch. In the position that produces the lower level output, the switch shunts part of the signal to ground so less reaches the amplifier. When the contacts are dirty or intermittent, the signal isn't shunted to ground and remains higher than the others.
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Old 5th January 2012, 03:48 PM   #12
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one last remaining issue with the class A picasso is i am only able to adjust the biasing pot between the range of 14mvdc - 19mvdc. i remember there being a wider range before 0-80mvdc. are the 500ohms pots i replaced the old ones with not the correct ones or is this normal and i should go ahead and max out the pots to get 19mvdc. this is across two different diodes. across the resistor at max i get 9.5mvdc.
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Old 5th January 2012, 07:01 PM   #13
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You won't hear a difference between 1mv and 20mv.

The turn-on threshold may be different on the replacement transistors so the bias control may not have enough range.
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Old 5th January 2012, 09:27 PM   #14
azvrt is offline azvrt  Netherlands
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Hi Perry, while I respect your opinion I have to disagree on the imaginary thing regarding op-amps.
This week we replaced the OP275 on two of the four channels with OPA2134.
This enables us to do back-to-back testing between channels 1+2 and 3+4 by simply moving the RCA's and speaker wires to the other channels. The source, wiring, speakers and music remain the same. The difference in sound is very apparent.

For instance on the Picasso the OP275 has more high frequency response and more low frequency responce than the OPA2134PA, which has strong mid frequency response. This alone affects the sound, and this is only one of several differences.

Thanks to the sockets we are using we can swap op-amps real fast on for instance the 2 channel Genesis amp for almost-immediate comparison of the music and yes the difference between op-amps is very noticable.

We had to spray some contact cleaner in the notorious Soundstream switches. The 2.0 / 5.0 volt input gain switches probably are the worst so we replaced them on that specific board since there was a lot of dust and corrosion in them.

I find it strange his board won't go beyond 19 mVDC bias. How much power supply voltage is the amplifier getting ? The bias will be different at 11, 12, 14 volts etc.
If, for instance, you are using a car battery as power supply on your test bench the voltage will drop well below 12 volts after powering on an amp, and so will the bias.

Recommended bias is 25 mVDC for this particular amplifier. The bias will be higher after a couple of minutes after being powered on. When you raise the left channel the right channel will also rise considerably, and vice-versa. On the Rubicon Class A's, it's opposite. When you raise the bias of the left channel, the bias of the right channel will decrease a little and vice-versa. Also, the bias on the Rubicons does not rise like it does on the Reference Picasso after being powered-on for a couple of minutes.

What is VERY important on Reference amplifiers is to retouch the driver PCB solderings. Especially on Picasso's they are very bad and can cause multiple problems. And the switches, like always. We very gently hit the housing of on of the stereo/bridged/mixed-mono switches on the Ref Picasso with the back of a screwdriver and the sound on one channel was cutting in and out. Contact spray cleaner and working the switches back and forth many times does the trick. Bad solderings on the driver boards were also causing problems on this board, but they are easily solved by applying some soldering to all of them, including the smaller control boards. By moving the driver PCB's gently around while the amplifier is playing music the sound on one channel might cut in and out and you might cause a temporary short circuit and see excessiver current draw on your test bench's power-supply. On this particular board one of the chokes had overheated so we replaced it and the nearby rectifiers just to be sure. It never hurts to replace the rectifiers on these Soundstreams.

The solderings on the Rubicons are better, generally.

Probably the top 3 problems on Reference and Reference s / sx amps are driver board solderings, switches and rectifiers, not necessarily in that order.
Rubicons have their own problems, though probably less of them.

Last edited by azvrt; 5th January 2012 at 09:33 PM.
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Old 6th January 2012, 02:04 AM   #15
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azvrt:
Try this. Have someone change them or leave them. You don't know. Sometimes they will change them, sometimes they will leave them as they are (you never know what's installed). Listen 10 times without having a clue as to which op-amp it is. Can you pick out the one that sounds so wonderful 100% of the time?

If they never change the op-amp, can you say that you would never say that the op-amp has been changed when it has not?

Would someone else that has never heard the amp be able to do the same?

If you can't do this without error 10 times or 100 times, the difference must not be pronounced and the belief that they sound much better must be imagined.

The only difference you can expect is a small decrease in the noise floor.

This assumes that the replacement op-amp is behaving properly and not creating something that you can key on (sometimes they have a faint oscillation).

This is a problem with experimenters. They expect to hear a difference so they do. They make a minor change and hear an incredible difference (soundstage just opened up, wonderful highs, tighter bass...). I'd bet that after they make 10 'upgrades' (all making an incredible difference, in their opinion) that they could not pick their super duper project from one with all original parts (this assumes that the original was properly designed by a competent design engineer). This is why I cannot participate in some of the forums. Too much snake oil being sold to unsuspecting newbies.

One example that stands out was an amp that was intentionally underbiased. It created a bit of crossover distortion. When listening to the underbiased amp, the amp had an 'airy' sound, more harmonics. This was especially pronounced with female vocals or a cello. In virtually all instances, the listeners thought the underbiased amp sounded better when it was actually more distorted.

Just because it's different doesn't mean it's more accurate or better (unless you like the distortion it adds). The op-amps aren't likely to add any distortion (but could if they're not stable in the circuit) but you can't rule it out unless you have the test equipment to verify it.
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Old 6th January 2012, 09:42 AM   #16
azvrt is offline azvrt  Netherlands
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Thanks for the suggestion, it makes sense to perform this test, yes. Will do.
I won't be checking for distortion (in most occasions) so if we do pass the test one might contribute it to distortion.
However, I must say that if a certain sound is pleasing to my ears and that sound turns out to be caused by a certain type of mild distortion, that's perfectly fine with me. It would be a shame to choose a technically more correct sound over a techically less correct but more pleasing (to the listener) sound. In other words the technically perfect flawless sound might not be the sound a certain person likes.

If the bass seems stronger with a stronger op-amp, maybe it can be proved with an oscilloscope ? For instance, play test tones at the same volumes at 70 Hz, 700 Hz and 7 kHz. If the op-amps are playing equally loud at 700 Hz but not at 70 Hz then it would be clear ?

In the end I won't be enjoying the music in my car with measuring equipment but with my ears.
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Old 6th January 2012, 02:08 PM   #17
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In my opinion, what's important is that you don't fall into the same mindset as other modders. If you want to deal with facts, you need to test in a way that gives definitive results.

It's unlikely that the low frequencies will be affected by the op-amps unless the previous op-amp was defective. Op-amps all work about the same with low frequencies.

If there is a difference at 70Hz, there is likely something wrong with the circuit.
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Old 6th January 2012, 07:41 PM   #18
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thanks for the input from everyone helping me resolve my issues. if i raise the bias to over 20mv i will hear difference in sound quality? if yes how will i go about doing so if i'm not able to turn it past 19mv? is it tip102/107 i need to replace, i am currently using on semi. Or is it ndp7050 (replaced with ndp7060)? fep16dt/fen16dt(replaced with fep16dta)? mc7815ct/mc7915ct (replaced with mc7815ctg/mc7915ctg)?

if the switches are switched to the 2v side all 4 channels play at same volume but if i move it over to the 5v i get that same problem of one of the channels playing louder than the rest. my headunit is sending a 5v out wouldn't i need it to be in the 5v position?

the bias doesn't raise to much after being powered on i believe i have the reference version its a blue board.
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Old 6th January 2012, 07:47 PM   #19
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You won't hear the difference with higher bias.

To make it go to greater bias with the current parts, you'd likely have to modify the circuit (change a couple of resistors).

In my opinion, you don't need to replace anything if it's producing clean audio.

The switch would only need to be in the 5v position if the amp reaches full power at a very low setting on the volume control. If you need to use it in the 5v position, you will either have to replace the switch or jump it out. For testing, you could solder jumpers between the pins that have the bad connection to confirm that it's the switch.
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Old 6th January 2012, 07:48 PM   #20
azvrt is offline azvrt  Netherlands
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You should try and find out whether you can hear a difference with the bias set to 19 mvdc and for instance 15 mvdc. If you canīt hear the difference, set it at 19 mvdc and donīt worry about it. Still, I find it strange you canīt set it any higher on that particular board.

I had the exact same problem you have with the 2v / 5v gain switches on a Ref 644s and 500sx. Replacing the switches (buy them from Jaime) should do the trick. You could also try contact spray cleaner which might also help.
At some point I replaced an old switch with a brand new one and it did not work properly ! Sprayed the new switch and never had problems since.

Blue boards are Reference. Very strange the bias doesnīt raise much while the amplifier warms up. Very strange you canīt set the bias any higher. I wish I could say something more useful but I donīt know the answers to that.
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