Hi All:
The Quad 606/909 comes with TLC271 opamp for its input duties and also feedback. Swapping this opamp for a OPA134 opens up the sound, tightens the bass BUT on shut-down of the amp, huge noises appear via the speakers which means either offset is high or some problem that I'm not sure of exists. Two number 0.1uF/100V Vishay 1837MKPs were attached directly on the PS pins for Rail-to-Gnd bypass.
Checking with a DMM on the speaker outputs show 18mV on switch-on while 19V on shut-down!!! with the OPA134. This really worries me.....
Quickly reverting to the Stock TLC271 opamp, the shut-down is absolutely noiseless. Usual turn-on 'thump' is present with Stock & the OPA134.
Does anyone recommend I attempt to swap the opamp with a more low & controlled offset one like the OPA602 or OPA606? Some recommended the OPA604 but my earlier experience with 2604 & its distortion kept me far away from this family.
Your suggestions will be most appreciated.
Thanks in advance.
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Arup
The Quad 606/909 comes with TLC271 opamp for its input duties and also feedback. Swapping this opamp for a OPA134 opens up the sound, tightens the bass BUT on shut-down of the amp, huge noises appear via the speakers which means either offset is high or some problem that I'm not sure of exists. Two number 0.1uF/100V Vishay 1837MKPs were attached directly on the PS pins for Rail-to-Gnd bypass.
Checking with a DMM on the speaker outputs show 18mV on switch-on while 19V on shut-down!!! with the OPA134. This really worries me.....
Quickly reverting to the Stock TLC271 opamp, the shut-down is absolutely noiseless. Usual turn-on 'thump' is present with Stock & the OPA134.
Does anyone recommend I attempt to swap the opamp with a more low & controlled offset one like the OPA602 or OPA606? Some recommended the OPA604 but my earlier experience with 2604 & its distortion kept me far away from this family.
Your suggestions will be most appreciated.
Thanks in advance.
--------------------
Arup
Hi
An alternative solution would be to ground the op amp inputs and then use a pair of transistors/fets as the input pair.
This was done by Musical Fidelity on the mk 2 versions of the P270 and A370 power amps.
Don
An alternative solution would be to ground the op amp inputs and then use a pair of transistors/fets as the input pair.
This was done by Musical Fidelity on the mk 2 versions of the P270 and A370 power amps.
Don
The op-amp in the 606 is only used for the dc servo. The rest of the circuitry is discrete. The TLC271 is a perfectly good op-amp for a dc servo. Certainly an OPA134 would be a foolish part to replace it with.
Many mod stores in the UK offer to change the opamp for better varieties like OPA227 or OPA134 and claim it makes a difference. There's been a debate that the Opamp in the 606/707/909 is "strictly for DC Servo" BUT
it does have its signature on the final output. I for instance, immediately noticed a complete change for the better. The Stock opamp is a slough in the bass dept that just slows it down. The bass definition is muddy. OPA134 just cleans up all this. So, with such an improvement, why wouldn't one use it. Only thing is that probably that Opamp is drawing more input current that upon shut-down is unable to offer the quiet decay.
606 / 909 Op Amp
I have had a lot of experiance with changing these op amps and can tell you the original or pehaps its 'B' version is the best to have. OPA134 cause the switch off bumps etc, the OPA227 can cause the power supply to lock up with a high DC offset and blown capacitors, particularly with low impeadence loads. Two customers have said the original sounds better then and 134 (better bass control) and I have to say I would agree. There are plenty of other componnets to get to work on in the 606, 707 and particulary later 909s.
I have had a lot of experiance with changing these op amps and can tell you the original or pehaps its 'B' version is the best to have. OPA134 cause the switch off bumps etc, the OPA227 can cause the power supply to lock up with a high DC offset and blown capacitors, particularly with low impeadence loads. Two customers have said the original sounds better then and 134 (better bass control) and I have to say I would agree. There are plenty of other componnets to get to work on in the 606, 707 and particulary later 909s.
Could I ask a silly question?
In the various Quad 606/707/909 schematics that are kicking around, pin 8 of the op amp appears to be connected to the output, pin 6. Now the TLC271 is slightly unusual in that it is "programmable", with pin 8 being used to select one of three "bias levels". Among other things, the GBW varies with bias setting.
From the data sheet:
Is there something fiendishly clever going on here, or is the output merely a convenient point to tie pin 8 to? Why have Quad used such a relatively unusual op amp?
(Assuming pin 8 really is connected to pin 6. Can anyone with a 606 confirm the PCB layout..?)
In the various Quad 606/707/909 schematics that are kicking around, pin 8 of the op amp appears to be connected to the output, pin 6. Now the TLC271 is slightly unusual in that it is "programmable", with pin 8 being used to select one of three "bias levels". Among other things, the GBW varies with bias setting.
From the data sheet:
Is there something fiendishly clever going on here, or is the output merely a convenient point to tie pin 8 to? Why have Quad used such a relatively unusual op amp?
(Assuming pin 8 really is connected to pin 6. Can anyone with a 606 confirm the PCB layout..?)
It is a CMOS opamp used as an integrator with a 2.2Meg input resistor and a 680nF feedback capacitor, giving a time constant of 1.5 seconds. The output of the op-amp is connected via a 120k resistor to the amp input. It can have no effect on the audio quality when the amplifier is operational, but a change of op-amp may well change the behaviour of the amplifier at switch-off, causing a nasty LF thump as the power rails collapse.
The Quad schematic certainly shows the two pins connected together by the way.
The Quad schematic certainly shows the two pins connected together by the way.
Thanks Ourboros.
I realise that the op amp has no effect on the sound, but any explanation as to why pin 8 is connected to pin 6 and not tied to a 'fixed' voltage? I don't suppose there's any chance at all that they are dynamically varying the op amp's characteristics at power up/down to prevent instability/'thump' etc..?
My work on the 606-II abt a year back led me to accept the fact that any deviation from the stock opamp, and you'll definitely get severe power up and down problems that can ruin your speakers too.
I have left the stock opamp in its place and have attacked at all its weaknesses in sound quality by performing other mods. All in all, the Quad amp sounds waaaaaayyy too good. No complaints whatsoever. Sad that some of the passive parts I used are no longer available (even if u throw your $$$).
Or else I would build up a 2nd 606-II with all the mods for the purpose of vertical bi-amping. (That ain't gonna happen anymore).
I have left the stock opamp in its place and have attacked at all its weaknesses in sound quality by performing other mods. All in all, the Quad amp sounds waaaaaayyy too good. No complaints whatsoever. Sad that some of the passive parts I used are no longer available (even if u throw your $$$).
Or else I would build up a 2nd 606-II with all the mods for the purpose of vertical bi-amping. (That ain't gonna happen anymore).
Yes, that aspect of it is baffling! The TLC271 has very different characteristics at the different bias settings, from a slew-rate of 3.6V/usec at high bias, down to only 0.05V/usec at low bias. Even though the output pin will only change its level very slowly, I don't see what is gained by this. If you were modifying the board, I would connect the bias pin to medium bias (OV).
By the way, the standard quad 606 uses simple shunt 6.8V zener diodes to provide the op-amp supply. If you fit an op-amp taking any more than 4mA supply current, the rails will go out of regulation. I know people sometimes add an NPN and a PNP buffer to the regulators to beef up the supply current, though you would need to change to 7.5V Zeners to keep the rails at the specified voltages.
By the way, the standard quad 606 uses simple shunt 6.8V zener diodes to provide the op-amp supply. If you fit an op-amp taking any more than 4mA supply current, the rails will go out of regulation. I know people sometimes add an NPN and a PNP buffer to the regulators to beef up the supply current, though you would need to change to 7.5V Zeners to keep the rails at the specified voltages.
606 op amps
Pin 6 and 8 are connected and I can only assume its to do with switch on and off behavior. I've also notices that in early 306s this connection is made with a wire link. So perhaps Quad found this a problem and solved it in quite an elegant way.
Pin 6 and 8 are connected and I can only assume its to do with switch on and off behavior. I've also notices that in early 306s this connection is made with a wire link. So perhaps Quad found this a problem and solved it in quite an elegant way.
there is something in the 405 mods ( snook or ludwig ) that talks of solving thumpish problems with new op amps. can't remember what now, but it's there, FYI, cheers, Paul🙂
Hi
Quads choice of the TLC271 is very strange indeed, the use of Pin 8 to pin 6 is to satisfy a bias signal, lazily without deploying a complex divider network. Reading the datasheet, its obvious the TLC271 is not designed for dual rail supplies, rather single supplies are where it is happiest existing. Studying the datasheets I draw the conclusion that it is a very fussy device with really not much going for it. Maybe its ability to be sensitive to small inputs might be its only redeeming feature. For instance describing the conditions where latch up might occur hardly brings confidence, -oh yes I really want some latch up today before lunch. .
So what does it do ? its really just a somewhat fancy buffer for the feedback with some High frequency rolloff, its equivalent is the capacitor C2 in a 405
The difficulty substituting other op amps that you are having is I think the connection to Pin 8 on that substituted device, Pin 8 is normally labelled as NC in a single op amp, but may never the less still allow improper connection, so you could try without Pin 8
You are welcome to try one of my AD825's with pass transistors which should control any turn on thump problems, in which case I can remove track for Pin 8 - send me a PM.
Cheers / Chris
Quads choice of the TLC271 is very strange indeed, the use of Pin 8 to pin 6 is to satisfy a bias signal, lazily without deploying a complex divider network. Reading the datasheet, its obvious the TLC271 is not designed for dual rail supplies, rather single supplies are where it is happiest existing. Studying the datasheets I draw the conclusion that it is a very fussy device with really not much going for it. Maybe its ability to be sensitive to small inputs might be its only redeeming feature. For instance describing the conditions where latch up might occur hardly brings confidence, -oh yes I really want some latch up today before lunch. .
So what does it do ? its really just a somewhat fancy buffer for the feedback with some High frequency rolloff, its equivalent is the capacitor C2 in a 405
The difficulty substituting other op amps that you are having is I think the connection to Pin 8 on that substituted device, Pin 8 is normally labelled as NC in a single op amp, but may never the less still allow improper connection, so you could try without Pin 8
You are welcome to try one of my AD825's with pass transistors which should control any turn on thump problems, in which case I can remove track for Pin 8 - send me a PM.
Cheers / Chris
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With a relay on the output you can have your cake (better sound) and eat it (control circuit to de-energize relay on power off). Just a thought. 😀
Ludwig adds a transistor in the positive supply. Snook use a smaller value series resistor in the negative supply.
Roger
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