Hi folks!
See the schematic of this chinese tube amplifier!
http://www.audioasylum.com/forums/tubediy/messages/26058.html
A volume pot inside a overall feedback loop!!!!
I think that we must going on designing ours owner amplifiers
as the French say "en est jamais mieux servie,que par soi meme"
Any coments?
Cheers
Jorge Santos
See the schematic of this chinese tube amplifier!
http://www.audioasylum.com/forums/tubediy/messages/26058.html
A volume pot inside a overall feedback loop!!!!
I think that we must going on designing ours owner amplifiers
as the French say "en est jamais mieux servie,que par soi meme"
Any coments?
Cheers
Jorge Santos
Chinese or not chinese!
Hi fedgrove!
If you see the post under the schematic the owner said that it is chinese.. i presume that is one more case of a chinese product sell in west with a western name...but that i don't know!
Regarding the volume pot is conected to C2 . C3 ...a 100KOhms pot!
Regards
Jorge Santos
Hi fedgrove!
If you see the post under the schematic the owner said that it is chinese.. i presume that is one more case of a chinese product sell in west with a western name...but that i don't know!
Regarding the volume pot is conected to C2 . C3 ...a 100KOhms pot!
Regards
Jorge Santos
The volume pot!!
Hi Joel!
I will past the folowup of the answer that i give to the guy!
Hi Foes!
The schematic of this chinese amplifier is seriousely wrong!!
First of all the junction point of R10,R11,R12 must be conected to ground!
Second and more important the volume control is inside the overall feedback loop...so when you turn the volume control the feedback try to compensate and the gain will be changed...because the feedback will see the variation of the volume control as a error!
A very much flowed design!
Regards
Jorge santos
Hi Joel!
I will past the folowup of the answer that i give to the guy!
Hi Foes!
The schematic of this chinese amplifier is seriousely wrong!!
First of all the junction point of R10,R11,R12 must be conected to ground!
Second and more important the volume control is inside the overall feedback loop...so when you turn the volume control the feedback try to compensate and the gain will be changed...because the feedback will see the variation of the volume control as a error!
A very much flowed design!
Regards
Jorge santos
Volume pot!
Joel Says! " i don't have a problem with the volume control placement though"
But the feedback loop have a problem!
Imagine when the operator turn the volume pot a little down...the feedback loop see this as a reduction of gain,and it try to compensate with a encrease of open loop gain...in the previous stages.
Resuming the open loop gain will be not defined but it changes with the volume pot position!Not a good designe pratice!
Regards
Jorge Santos
PS: In the link you can see the complaint of the owner saying that the amp is saturating!
Joel Says! " i don't have a problem with the volume control placement though"
But the feedback loop have a problem!
Imagine when the operator turn the volume pot a little down...the feedback loop see this as a reduction of gain,and it try to compensate with a encrease of open loop gain...in the previous stages.
Resuming the open loop gain will be not defined but it changes with the volume pot position!Not a good designe pratice!
Regards
Jorge Santos
PS: In the link you can see the complaint of the owner saying that the amp is saturating!
Are we trying to interpret the circuit operation in a different way to the designer? While it appears an inefficient method, is this pot actually there to control the level of feedback? At full on, there is max feedback. Perhaps on different inputs the user is supposed to alter the control to suit personal taste. Turning the control back will not be particularly effective at reducing volume as the reduction in feedback increases amplification. After all, vol and tone controls could appear in a pre-amp which is used with the amp. If you place the vol control in the main amp then why not your bass/treble controls also?
Perhaps this is a cunning amp designed by inscruitable designers to fool decadent western capitalist audio fans?
Perhaps this is a cunning amp designed by inscruitable designers to fool decadent western capitalist audio fans?
Hi Bournville!!
Se also:
http://www.audioasylum.com/forums/tubediy/messages/26132.html
The pot is efectively a volume pot!
regards
Jorge Santos
Se also:
http://www.audioasylum.com/forums/tubediy/messages/26132.html
The pot is efectively a volume pot!
regards
Jorge Santos
Yes, it may act as a volume control, but it may not have been designed for that function, but to control the level of feedback.
We've agreed that putting a vol control in a feedback loop is bad practice, so looking at the circuit it appears wrong. However, the designer may have wanted to give the listener some degree of control over the level of feedback (for whatever reason) so the pot was placed there for that purpose.
It's not what we would do, but who knows what was in the head of the designer? Incidentaly, it's not the first time I've seen a vol control within a feedback loop!
We've agreed that putting a vol control in a feedback loop is bad practice, so looking at the circuit it appears wrong. However, the designer may have wanted to give the listener some degree of control over the level of feedback (for whatever reason) so the pot was placed there for that purpose.
It's not what we would do, but who knows what was in the head of the designer? Incidentaly, it's not the first time I've seen a vol control within a feedback loop!
The saga of the pot!
I think that if the intention of the designer is to change the level of the feedback it will must have a resistor betwen the ground and the terminal of the pot that is now conected to ground.
For when the pot is in the minimum position the amp don't shut of!
I'm not surprised that you a i also see pots inside feedback loops
...afterall there are a lot of bad designes out there
Regards
Jorge Santos
I think that if the intention of the designer is to change the level of the feedback it will must have a resistor betwen the ground and the terminal of the pot that is now conected to ground.
For when the pot is in the minimum position the amp don't shut of!
I'm not surprised that you a i also see pots inside feedback loops
...afterall there are a lot of bad designes out there
Regards
Jorge Santos
One thing that worries about this amp is the way the EM80 'magic eye' indicator is connected - if it was meant to indicate volume levels surely it would be after this pot?
Alternatively, if the pot is a feedback control of some kind and the EM80 indicated the level of feedback (minimum display being the maximum feedback) then it would work in this way... (I admit I'm not an expert on 'magic eyes' - but why have one in an amp anyway?)
An alternative explanation to the whole problem might be a slight copying mistake - if the pot had a small cap in its connection to ground it would function as a tone control, which although regarded by many as bad practice is much more common to find in feedback loops. Then the EM80 simply indicates overall volume levels.
I would like to know how old this design is - EL41s are not widely used these days in preference to EL84s.
Alternatively, if the pot is a feedback control of some kind and the EM80 indicated the level of feedback (minimum display being the maximum feedback) then it would work in this way... (I admit I'm not an expert on 'magic eyes' - but why have one in an amp anyway?)
An alternative explanation to the whole problem might be a slight copying mistake - if the pot had a small cap in its connection to ground it would function as a tone control, which although regarded by many as bad practice is much more common to find in feedback loops. Then the EM80 simply indicates overall volume levels.
I would like to know how old this design is - EL41s are not widely used these days in preference to EL84s.
My damned amp
Hi all,
I'm the owner af the amplifier.
THanks to Jorge to share my problem with you all
Believe it or not, the volume amplifier works.
The EM80 works too, and indicate the input level. Infortunately, the input sensitivity is to high and the amplifier tends so saturate for every volume position... If someone can help me to calculate R4 (and R2) to lower the sensitivity, be welcome ! Also if it is possible de lower the feedback, you can explain...
The shematics shows EL41, in fact this amplifier exist also in EL90 (my amplifier) and in EL84 version. The design seems to be from 1998 !?
Rgrds,
Gauthier
Hi all,
I'm the owner af the amplifier.
THanks to Jorge to share my problem with you all
Believe it or not, the volume amplifier works.
The EM80 works too, and indicate the input level. Infortunately, the input sensitivity is to high and the amplifier tends so saturate for every volume position... If someone can help me to calculate R4 (and R2) to lower the sensitivity, be welcome ! Also if it is possible de lower the feedback, you can explain...
The shematics shows EL41, in fact this amplifier exist also in EL90 (my amplifier) and in EL84 version. The design seems to be from 1998 !?
Rgrds,
Gauthier
My damned amp
Hi all,
I am the owner of this damned amplifier.
Thanks to Jorges to have exposed my problem.
First of all, believe it of not, it works not so bad. Volume works (Yes !!) and the EM80 indicate the input level...
The design is from 1998. The amplifier exist in 3 ouput tube version : EL41-EL90 (my amplifier) and EL84. Also, the quality of the components is not so high... (chinese tubes and carbo resistors...)
Well, input sentitivity is to high, but i guess it's possible to lower it by playing with R4 (and R2 for correct bias ?). How to calculate it ? How to lower feedback too (i know, it will raise global gain but if first stage gain is lower ...) Any Idea ??
Regards,
Gauthier
Hi all,
I am the owner of this damned amplifier.
Thanks to Jorges to have exposed my problem.
First of all, believe it of not, it works not so bad. Volume works (Yes !!) and the EM80 indicate the input level...
The design is from 1998. The amplifier exist in 3 ouput tube version : EL41-EL90 (my amplifier) and EL84. Also, the quality of the components is not so high... (chinese tubes and carbo resistors...)
Well, input sentitivity is to high, but i guess it's possible to lower it by playing with R4 (and R2 for correct bias ?). How to calculate it ? How to lower feedback too (i know, it will raise global gain but if first stage gain is lower ...) Any Idea ??
Regards,
Gauthier
Have a Mars bar, Bournville - you're right
We're looking at this the wrong way. We see a push-pull output stage with loudspeaker impedances written next to the transformer, assume that it's a loudspeaker amplifier, and chortle at the poor (Hi-Fi) design. But what if it started life as a musician's effects unit? The EM81 would indicate how much compression/overdrive was being applied - allowing repeatable effects...
We're looking at this the wrong way. We see a push-pull output stage with loudspeaker impedances written next to the transformer, assume that it's a loudspeaker amplifier, and chortle at the poor (Hi-Fi) design. But what if it started life as a musician's effects unit? The EM81 would indicate how much compression/overdrive was being applied - allowing repeatable effects...
Nice to have you here with us
Hi foes!
I belive that the first step is to put the pot at the imput...after that i'm sure that the problem of overload of the imput will desapear..
After changing the pot for to lower the feedback raise the value of R14
Cheers
Jorge Santos
Ps. Sincerly wellcome!
Hi foes!
I belive that the first step is to put the pot at the imput...after that i'm sure that the problem of overload of the imput will desapear..
After changing the pot for to lower the feedback raise the value of R14
Cheers
Jorge Santos
Ps. Sincerly wellcome!
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