Hi,
I have acquired an solid state amplifier. On the front panel it said:
Monogram Professional Audio
3100
class a+
studio reference
handcrafted in England
It is the usual rack-mount size width and would occupy one and a half rack spaces. The interior shows the power supply (I guess +/- 50v) in the middle, and two amp modules at the sides. There are only 2 TO-3 transistors each side (2n3773).
I converted the amp back to use 220v mains last week. Now it is singing but I have found some problems in the amp.
1/ there is a hum/buzz heard, not from the speakers, not from the power supply, but from the amp modules. It is only barely audible at a distance of 3 feet. I could not find any part inside which is vibrating.
2/ the TO-3 transistors do not feel hot. In fact it is only a little bit warm to touch, may be about the same as body temperature. The heatsinks are even cooler. In contrast, the drivers and the heatsinks are very hot. I found this very puzzling as the amp claimed to be class A+ but yet feel much cooler than my NAD 302. There is no infomation on the amp about the power output, but I guess it is at most 20w from the 2 transistors.
3/ ccassionally, a tick is heard from the left channel speaker.
I would be most grateful if you could kindly tell me more about this Monogram amp or give me some suggestions to solve these problems.
Thank you for your time to read this post.
Best wishes,
Russell SIT
I have acquired an solid state amplifier. On the front panel it said:
Monogram Professional Audio
3100
class a+
studio reference
handcrafted in England
It is the usual rack-mount size width and would occupy one and a half rack spaces. The interior shows the power supply (I guess +/- 50v) in the middle, and two amp modules at the sides. There are only 2 TO-3 transistors each side (2n3773).
I converted the amp back to use 220v mains last week. Now it is singing but I have found some problems in the amp.
1/ there is a hum/buzz heard, not from the speakers, not from the power supply, but from the amp modules. It is only barely audible at a distance of 3 feet. I could not find any part inside which is vibrating.
2/ the TO-3 transistors do not feel hot. In fact it is only a little bit warm to touch, may be about the same as body temperature. The heatsinks are even cooler. In contrast, the drivers and the heatsinks are very hot. I found this very puzzling as the amp claimed to be class A+ but yet feel much cooler than my NAD 302. There is no infomation on the amp about the power output, but I guess it is at most 20w from the 2 transistors.
3/ ccassionally, a tick is heard from the left channel speaker.
I would be most grateful if you could kindly tell me more about this Monogram amp or give me some suggestions to solve these problems.
Thank you for your time to read this post.
Best wishes,
Russell SIT
A slip of the Keyboard.
those who have to write out the advertises (low payed employees)
can slip on the keyboard.
Such things happen easy, when you are in a hurry.
This can explain why the "B" of Class AB is so frequently absent.
Hope I didn't slip
/halo
Sometimes, due to the human factor,Nelson Pass said:
those who have to write out the advertises (low payed employees)
can slip on the keyboard.
Such things happen easy, when you are in a hurry.
This can explain why the "B" of Class AB is so frequently absent.
Hope I didn't slip
/halo
Can we help you? Russell SIT
what keyboard do you have, JoeBob
First,
Are you satisfied with what you got for your money?
Did you get what you thought you paid for?
In sweden we have an authority
that handles complaint from us costumers.
They can enforce public and/or legal pressure
onto those trying to fool people.
The amplifer does not have to be bad, because it is not a Class A.
Can you or any of your friends have any use for the amp.
Then it might not be all wasted money.
If you decide to keep the amp, we will help you, for sure,
to get it working as good as possible.
That means a great lot,
as the sum of knowledge at this diyAudio Forum
is hard to match anywhere in the world.
Sincerely
/halojoy
from a northern land
close?JoeBob said:
what keyboard do you have, JoeBob
What can I give for advice to our Hong Kong friend in the eastern land?
First,
Are you satisfied with what you got for your money?
Did you get what you thought you paid for?
In sweden we have an authority
that handles complaint from us costumers.
They can enforce public and/or legal pressure
onto those trying to fool people.
The amplifer does not have to be bad, because it is not a Class A.
Can you or any of your friends have any use for the amp.
Then it might not be all wasted money.
If you decide to keep the amp, we will help you, for sure,
to get it working as good as possible.
That means a great lot,
as the sum of knowledge at this diyAudio Forum
is hard to match anywhere in the world.
Sincerely
/halojoy
from a northern land
Greetings from the Far East
Thanks for the jokes and responses 🙂
The amp does have pretty large heatsinks and power supply. There are 5 trimpots on each board. Could anyone tell me what I should do to re-bias it to run in class A?
Thanks in advance for your kind help.
Greetings from Far East,
Russell SIT
Thanks for the jokes and responses 🙂
The amp does have pretty large heatsinks and power supply. There are 5 trimpots on each board. Could anyone tell me what I should do to re-bias it to run in class A?
Thanks in advance for your kind help.
Greetings from Far East,
Russell SIT
Idle Current Setting
Find a resistor that the output stage current have to pass.
Near those biggest transistors.
Try to figure out how what ohms that resistor is. Usually <1.0 Ohm
Idle Current = VoltResistor/ohms. I=U/R
Then comes finding out which trimpot controls/set that current.
Usually that trimmer is between the output transistors (Biggest)
Can also be between 2 smaller transistors.
First you have to know how much current flows.Russell Sit said:
Find a resistor that the output stage current have to pass.
Near those biggest transistors.
Try to figure out how what ohms that resistor is. Usually <1.0 Ohm
Idle Current = VoltResistor/ohms. I=U/R
Then comes finding out which trimpot controls/set that current.
Usually that trimmer is between the output transistors (Biggest)
Can also be between 2 smaller transistors.
With 5 pots to choose from, I recommend that you
make an effort to get a service manual or schematic
before diving in.
make an effort to get a service manual or schematic
before diving in.
I suspect that the Output Transistors have failed. Check this aspect and replace them with either MJ15003 or MJ21193. The 2N3773 are meant for Power Supplies and do not meet audiophile specs.
If you find that the output transistors need replacement, do that first and do not touch any of the 5 trimpots to start with. Measure the quiscent current and let us know. It will help, if you could tell us what driver transistors etc., are used.
Cheers,
If you find that the output transistors need replacement, do that first and do not touch any of the 5 trimpots to start with. Measure the quiscent current and let us know. It will help, if you could tell us what driver transistors etc., are used.
Cheers,
On a closer look to the circuit boards, there are actually 4 labelled trimpots each side. They are
V1 gain
V2 +lim
V3 -lim
V4 Iq
All of the resistors on the boards are low wattage type, except 2 next to V4. On the resistors I saw R20 5% CB5. I guess they are 20ohm resistors, may be 5 watts rated.
I have also done some crude measurements on the amp. The power supply is +/- 65v. When there is no signal in, the voltage across the 20ohm resistors is not recordable (? 0 volt) (? class B operation).
I guess V4 is the control of the idle current. However, if the higher wattage resistors is for gauging the idle current, the maximum current through it will be limited to a small value by the 20ohm resistor. In addition, there are 2 of them each side, and I do not know on which one I should base my measurement.
There is a sticker beneath the left circuit board with a date of 17th Sept, 1978, so this is a rather old amp. From the size of the heatsinks and the power supply (2 capacitors of 10000uF, 63V and a big EI transformer), to make it running in class A is not difficult when it was manufactured. I tend to believe that it was originally made to run in class A, but was somehow modified later to give a higher power output (class B now?).
Best wishes,
Russell SIT
To Sam: thank you for your suggestion. The amp does sound OK and there is no balance problem. Therefore though I cannot rule out the possibility of bad outputs, it is not high on my list of differential diagnosis.
V1 gain
V2 +lim
V3 -lim
V4 Iq
All of the resistors on the boards are low wattage type, except 2 next to V4. On the resistors I saw R20 5% CB5. I guess they are 20ohm resistors, may be 5 watts rated.
I have also done some crude measurements on the amp. The power supply is +/- 65v. When there is no signal in, the voltage across the 20ohm resistors is not recordable (? 0 volt) (? class B operation).
I guess V4 is the control of the idle current. However, if the higher wattage resistors is for gauging the idle current, the maximum current through it will be limited to a small value by the 20ohm resistor. In addition, there are 2 of them each side, and I do not know on which one I should base my measurement.
There is a sticker beneath the left circuit board with a date of 17th Sept, 1978, so this is a rather old amp. From the size of the heatsinks and the power supply (2 capacitors of 10000uF, 63V and a big EI transformer), to make it running in class A is not difficult when it was manufactured. I tend to believe that it was originally made to run in class A, but was somehow modified later to give a higher power output (class B now?).
Best wishes,
Russell SIT
To Sam: thank you for your suggestion. The amp does sound OK and there is no balance problem. Therefore though I cannot rule out the possibility of bad outputs, it is not high on my list of differential diagnosis.
A+ really = B
Because the power supply rails are as high as +/- 65 volts I suspect that it isn't really a true class A amp. 65 v peak into 8 ohms resistive is a little over 7.5 amps peak so you have to have a standing current of at least 7.5 amps to swing down to -65 volts into 8 ohms resistive. 130v x 7.5 amps x 2 channels = 1950 watts dissipation. 😱 Your power tranny would have to be 3kVa minimum. 500w per transistor. The power supply ripple would be 10v per amp per 1000uF at 50Hz (100Hz ripple) = 15 volts p/p ripple. Even in class B those transistors would be getting a safe operating area thrashing. I think 30v would be stacks, at about 3.5 amps per channel. Just check the transistor ratings.
Because the power supply rails are as high as +/- 65 volts I suspect that it isn't really a true class A amp. 65 v peak into 8 ohms resistive is a little over 7.5 amps peak so you have to have a standing current of at least 7.5 amps to swing down to -65 volts into 8 ohms resistive. 130v x 7.5 amps x 2 channels = 1950 watts dissipation. 😱 Your power tranny would have to be 3kVa minimum. 500w per transistor. The power supply ripple would be 10v per amp per 1000uF at 50Hz (100Hz ripple) = 15 volts p/p ripple. Even in class B those transistors would be getting a safe operating area thrashing. I think 30v would be stacks, at about 3.5 amps per channel. Just check the transistor ratings.
Are you sure it is +/- 65vdc and not only 65vdc,at that time 65vdc
was popular for amp coming from England.Is it possible to get
other transistors number used.
was popular for amp coming from England.Is it possible to get
other transistors number used.
I was just taking stock of some of my "repair projects" purchased over the years very cheap on ebay and so was googling about my Monogram 3000/3100 pre-power. I think mine has distortion on one channel - can't remember.
Anyway, they came with some circuit diagrams of the main amp circuit - one hand-drawn and the other printed. If anyone wants a scan (they're not great quality as it is) I'd be happy to oblige and scan/send them on (or post up here).
Here is some of the blurb from the sales sheet I have too (might explain the "+" in Class A):
Capital letters are theirs, all typos are mine.
Some selected specs:
Power Output: 3100 : 105W RMS
Damping factor: >400 to 1KHz - 8 ohms
Input impedance: 22K at min. level 15K at max. level.
Sensitivity: 0.775 RMS 0dB/1.5 volt max. Internally adjustable
Stability: Unconditionally stable with any load or source impedance
Electronic Design: Pure Class A at 1 amp, class AB at 1amp - 5amps.
(funny how class A gets a "Pure" and capitals, class AB is in lower case).
I have a hifi mag from about 78 or so with a list of products/specs/prices. The Monogram pre-power (3000/3100) was about £420 (from memory).
Anyway, they came with some circuit diagrams of the main amp circuit - one hand-drawn and the other printed. If anyone wants a scan (they're not great quality as it is) I'd be happy to oblige and scan/send them on (or post up here).
Here is some of the blurb from the sales sheet I have too (might explain the "+" in Class A):
Capital letters are theirs, all typos are mine.
Some selected specs:
Power Output: 3100 : 105W RMS
Damping factor: >400 to 1KHz - 8 ohms
Input impedance: 22K at min. level 15K at max. level.
Sensitivity: 0.775 RMS 0dB/1.5 volt max. Internally adjustable
Stability: Unconditionally stable with any load or source impedance
Electronic Design: Pure Class A at 1 amp, class AB at 1amp - 5amps.
(funny how class A gets a "Pure" and capitals, class AB is in lower case).
I have a hifi mag from about 78 or so with a list of products/specs/prices. The Monogram pre-power (3000/3100) was about £420 (from memory).
I posted the questions nearly 12 years ago. It is hard to believe that there is indeed a kind soul who could offer a schematic to help eventually.
If it is not too troublesome for you, I would be most grateful if you can post the schematics here.
Thanks a lot for your very kind help! I am going to dig up the amp from my pile of unfinished stuffs now.
If it is not too troublesome for you, I would be most grateful if you can post the schematics here.
Thanks a lot for your very kind help! I am going to dig up the amp from my pile of unfinished stuffs now.
Hehe.. Yes 12 years is a long time to wait (shows how long DIY audio has been around and also how great and stable their forum is! - many lesser forums have ditched old posts when I look back..).
I'll scan what I have when I'm next back home (could be tonight or tomorrow) including the promotional leaflets and post them on here for prosperity.
I'll have to see what is wrong with mine - I'm sure I bought it with a problem as it was only £35 or so a few years ago.. (a bargain for the pre-amp to be included too). Shame they use 1/4 jacks between pre and power - might have to change that .
They sound like they could be pretty good amplifiers in a retro sense. Certainly they were pretty expensive for the time.
The diagram does show +65v and -65v rails by the way.
I'll scan what I have when I'm next back home (could be tonight or tomorrow) including the promotional leaflets and post them on here for prosperity.
I'll have to see what is wrong with mine - I'm sure I bought it with a problem as it was only £35 or so a few years ago.. (a bargain for the pre-amp to be included too). Shame they use 1/4 jacks between pre and power - might have to change that .
They sound like they could be pretty good amplifiers in a retro sense. Certainly they were pretty expensive for the time.
The diagram does show +65v and -65v rails by the way.
Hi All!
I've just got myself (hopefully!) a bargain Monogram Capricorn power amplifier off eBay, for the princely sum of £40....
It seems to work, but I can't seem to find anything much about it. I've taken the lid off to check for any cap leaks/obvious issues, and taken some photos of it's insides.
I was wondering if anyone knew about the Capricorn amplifier - the few things I can find about Monogram (aka Don Purkis) seem to only speak of the 3000 series amplifiers. The circuit board proclaims it as a 'Class A+' amplifier, so thought this may be the thread to ask!
I think it sounds OK, but obviously it's an old piece of kit, and may need a service/recapping at points, so if anyone has a schematic that could be useful.
Was it ever regarded as a good amplifier?
Hope you guys can help, and expand on the miniscule amount of information that seems to be available on Monogram 🙂
Cheers!
Chris.
I've just got myself (hopefully!) a bargain Monogram Capricorn power amplifier off eBay, for the princely sum of £40....
It seems to work, but I can't seem to find anything much about it. I've taken the lid off to check for any cap leaks/obvious issues, and taken some photos of it's insides.
I was wondering if anyone knew about the Capricorn amplifier - the few things I can find about Monogram (aka Don Purkis) seem to only speak of the 3000 series amplifiers. The circuit board proclaims it as a 'Class A+' amplifier, so thought this may be the thread to ask!
I think it sounds OK, but obviously it's an old piece of kit, and may need a service/recapping at points, so if anyone has a schematic that could be useful.
Was it ever regarded as a good amplifier?
Hope you guys can help, and expand on the miniscule amount of information that seems to be available on Monogram 🙂
Cheers!
Chris.
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