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Old 10th January 2013, 04:42 PM   #21
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Thank you for the nice references! I think I will consider Self's book you mention.

So I should try to put that circuitry before the speaker to mute things, if I understand well.

Jaycee, I think I have learned this at last! I will see which option serves me better - the mechanical on-off muting switch or a relay switch kicking in after being fed through an RC stage. A matter of fitting to the existing space.
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Old 10th January 2013, 05:05 PM   #22
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Err..no.
The JFET switch is a low signal level device for the amplifier input. It mutes all signal sources and keeps the input at a low impedance to prevent spurious
noise pickup. This is how muting circuits work generally. You even find them in countless small powered monitor speakers that are now sold everywhere.

The speaker relay certainly mutes everything but it is not technically a muting switch. More a circuit-breaker!
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Old 10th January 2013, 05:44 PM   #23
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Quote:
Originally Posted by Ian Finch View Post
Err..no.
The JFET switch is a low signal level device for the amplifier input. It mutes all signal sources and keeps the input at a low impedance to prevent spurious
noise pickup. This is how muting circuits work generally. You even find them in countless small powered monitor speakers that are now sold everywhere.

The speaker relay certainly mutes everything but it is not technically a muting switch. More a circuit-breaker!
That seems quite logical. To tell you the truth, I was searching for the second point to make the circuit breaker - didn't notice there wasn't one!

Actually, isn't that was I am supposed to do (a simple circuit breaker)? Just cut these pops off the speaker, since they are there for the cap charges? I have a 1k resistor across the speaker, which should help the output cap charge. And then connect the speaker.
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Old 19th March 2013, 02:33 PM   #24
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Default Weird noise characteristics

I noticed something weird: when turning volume lower than 9 o'clock, thermal noise through the speaker is increased, compared to its level when I use it above that point. Seems like with volume rolled off, I get maximum noise.
All these observations with muted input signal. Any opinions?
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Old 19th March 2013, 11:04 PM   #25
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So how does the power amp sound distorted?
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Old 19th March 2013, 11:47 PM   #26
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By "thermal noise", the OP is likely referring to hiss - also likely with his circuit arrangements.
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Old 20th March 2013, 01:02 AM   #27
JMFahey is offline JMFahey  Argentina
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Can't say the design is *wrong*, becauseit's not, but you made a couple choices which may shift it towards some instability.
Maybe "just one" would do nothing, but 2 or 3 combined yes.
In no particular order:

1) you chose a too low value for the presence pot.
Remember that no matter where you set it, it is *always* in circuit and padding down both the input attenuator net (4K7/4K7) and the feedback network (also 4K7/4K7) .
With the pot in the middle ("on 5"), you have 1/2 pot to ground (yes, at a certain frequency and through the gyrator) on each side, meaning each grounded 4K7 has 500 ohms in parallel.
NOTE: next time identify each part , as in R1 , R2 , etc. for clarity.
So the input signal has an extra unneeded 20dB attenuation, even when set to "flat" and at the same time the recovery stage has same extra unneeded 20dB boost ... **always**
Then you will hear some hiss you don't know where it comes from ... although the schematic will *simulate* well, have the proper curves, etc.
Usual practice is to choose a pot value 5X to 10X that of input and feedback resistors.

2) you are asking too much from that presence circuit, those 47r resistors are way too low, and worsen what I mentioned above.
Yes, I think you "had" to choose them because otherwise range would have been too weak .... but this comes fron the earlier poor choice of presence pot.

3) you did not place an output cap, although the output sits at around +15V.

4) why a constant current source at the output BC549?
What's wrong with a good old 10K resistor to ground?
Constant current sources such as this one, really are a tight feedback closed loop gain block, set to keep current constant.
Being a closed loop gain block means it "can" oscillate, if conditions are proper.
This will never happen with a plain resistor.

5) don't know why, but often Gyrator driven EQs oscillate or at least are unstable, although gain or requirememnts don't seem extreme.
I found 470pF or 1000pF across the lower LF353 feedback resistor do not change sound in an audible way but kill this tendency for good.

6) this is not your fault or anybody's: Op Amps are often "stupid" at very low rail voltages, remember that normal ones can *not* reach rails by 1.5 to 2.5V , so at such voltages outputs can't swing what's asked by feedback networks and can do so "spasmodically", meaning oscillating wildly.
Some of my preamps oscillated with a rising "squiiiiiiikkkk" sound when amp was turned off.
Didn't worry too much, respected Gallien Krueger amps did the same
Later, like anybody else, added a mute.

So be happy, your preamp design is sound, but needs a little tweaking .... like most others
Specially at the prototype stage.
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Old 20th March 2013, 09:39 AM   #28
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Quote:
Originally Posted by Ian Finch View Post
By "thermal noise", the OP is likely referring to hiss - also likely with his circuit arrangements.
Well, exactly that - the resistors are carbon film, too.

Quote:
Originally Posted by Printer2 View Post
So how does the power amp sound distorted?
Unfortunately, I don't think I ever managed to overdrive the power stage. In the beginning, I had tremendous gain and the opamps clipped, lowered that and now I get a pretty much clean signal.
To my ears, combined with a Phillips HP8 12' speaker, it sounds rather musical for a solid state (guitar) amp.
Quote:
Originally Posted by JMFahey View Post
Can't say the design is *wrong*, becauseit's not, but you made a couple choices which may shift it towards some instability.
Maybe "just one" would do nothing, but 2 or 3 combined yes.
In no particular order:

1) you chose a too low value for the presence pot.
Remember that no matter where you set it, it is *always* in circuit and padding down both the input attenuator net (4K7/4K7) and the feedback network (also 4K7/4K7) .
With the pot in the middle ("on 5"), you have 1/2 pot to ground (yes, at a certain frequency and through the gyrator) on each side, meaning each grounded 4K7 has 500 ohms in parallel.
NOTE: next time identify each part , as in R1 , R2 , etc. for clarity.
So the input signal has an extra unneeded 20dB attenuation, even when set to "flat" and at the same time the recovery stage has same extra unneeded 20dB boost ... **always**
Then you will hear some hiss you don't know where it comes from ... although the schematic will *simulate* well, have the proper curves, etc.
Usual practice is to choose a pot value 5X to 10X that of input and feedback resistors.

2) you are asking too much from that presence circuit, those 47r resistors are way too low, and worsen what I mentioned above.
Yes, I think you "had" to choose them because otherwise range would have been too weak .... but this comes fron the earlier poor choice of presence pot.

3) you did not place an output cap, although the output sits at around +15V.

4) why a constant current source at the output BC549?
What's wrong with a good old 10K resistor to ground?
Constant current sources such as this one, really are a tight feedback closed loop gain block, set to keep current constant.
Being a closed loop gain block means it "can" oscillate, if conditions are proper.
This will never happen with a plain resistor.

5) don't know why, but often Gyrator driven EQs oscillate or at least are unstable, although gain or requirememnts don't seem extreme.
I found 470pF or 1000pF across the lower LF353 feedback resistor do not change sound in an audible way but kill this tendency for good.

6) this is not your fault or anybody's: Op Amps are often "stupid" at very low rail voltages, remember that normal ones can *not* reach rails by 1.5 to 2.5V , so at such voltages outputs can't swing what's asked by feedback networks and can do so "spasmodically", meaning oscillating wildly.
Some of my preamps oscillated with a rising "squiiiiiiikkkk" sound when amp was turned off.
Didn't worry too much, respected Gallien Krueger amps did the same
Later, like anybody else, added a mute.

So be happy, your preamp design is sound, but needs a little tweaking .... like most others
Specially at the prototype stage.
Thank you very much for all your observations! Seems like they could be dead on.

I will try to respond to some of these:

(1) Just used what was laying around - I had a 1k pot at hand, so I used it. I think I saw another 10k linear one somewhere, so I maybe use this. And, really sorry for the lack of resistor markings, I had no time to add them. I will post a complete schematic when everything is finished!

(2) The 47R resistors do not to limit the range as you say - and they were almost essential, because I must admit that this presence pot is the one I use most and very happily. Should I have this 10k pot, I will make them bigger too!

(3) What is the point of the circuit you are referring to? The output of the BC549C? Because this is followed by a 100n input cap of the power stage.

(4) Just had some transistors around - decided to use them for fun. I know it may be no fun at all, but I had no audible problems yet. I will maybe try the simple 10k.

(5) Combined with (6), it seems to be the final cause that this thread started. I will give that cap a try, thanks for the advice! Even though the "charging" noise is now gone, with the addition of a 78L15. Sadly, I have not updated the schematic yet.

Again, you are not obliged to have read all previous posts, but should you do so, you will realise that the build started as a subtle experimentation.

I will try to find some time modify the preamp, then!

Last edited by audiostrat; 20th March 2013 at 09:42 AM.
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Old 20th March 2013, 02:12 PM   #29
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One interesting observation: as I mentioned, the hiss becomes greatest when volume is at 0. When volume is at maximum, the hiss is of greater amplitude (logical) but has greater high frequency content, compared to the hiss at minimum volume. Again, when volume is in the middle position, I get better hiss compared to the two aforementioned cases.

I observed that the hiss at zero volume remains the same, no matter the position of any other pot. And when I short-circuited the input of the power amp, no hiss at all.

Seems like it could come from the buffer section?
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Old 20th March 2013, 04:21 PM   #30
JMFahey is offline JMFahey  Argentina
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Hi audiostrat.
Respect to the output cap, I also thought you would rely on the power amp input cap, and it works, but it's "good manners" putting the proper one at the preamp output (if you have DC there) so the preamp can be used with any other amp, or to go straight into a mixer, drive effects, whatever.
As of the hiss with the volume poy at "0" it indicates some grounding problem.
That explais hiss on 10 (normal, comes from the earlier stages), "0", because of grounding, and less at intermediate points because you are neither on 10 nor touching ground.
As an experiment, lift the Volume ground connection and with a piece of "flying" wire, grounding at the output jack ground pin.
*Should* work, although this kind of problems is never 100% certain.
Proper grounding is a sort of black art.
Good luck

By the way, you are the second Greek I answer to today, maybe that means I'll have to visit beautiful Greece again??

(Hope so )
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