I have always had a soft spot for a Dynaco Stereo 120 solid state power amp. It was the first solid state amp with real power in wide production. It did an awful lot with just 6 transistors per channel. Still, it probably did a lot of damage, too.
The output stage was notoriously under-biased, creating lots of crossover distortion at low levels. If you measured the distortion and didn't look at or think about the harmonics, the numbers seemed pretty good...in reality, it was pretty miserable classic notch (crossover) distortion...
Of course, if you let it warm up long enough, the bias current would creep up, and some of the worst distortion wouldn't be quite so bad...Perhaps this one amp is responsible for a world of audiophiles warming up equipment for hours before listening!
Then, there were a few out there, that probably sounded great, but the output stage bias was on the hairy edge...play them loud enough, long enough, and the magic musical smoke would escape from the amp as the quiescent current ran away...
Anyway, with this major shortcoming in mind (purists might point to a number of other shortcomings...), I designed an analog bias controller for the output stage quiescent current. It nails the output stage quiescent current wherever you'd like (I've settled on around 50 mA), and keeps it from varying over temperature, no matter how hard you beat on the amp.
You can kluge it onto existing working amps with very little fuss. The result is distortion behavior much like a tube amp...almost no distortion at low levels (e.g. unmeasurable harmonics with 10 mV rms input), and then as output amplitude picks up, you get more 2nd harmonic than 3rd. The distortion increases gradually until you approach clipping.
I figured I would just post this much to see if there's interest...if so, I can write up the circuit, and post some information...
All for now...comments invited...
Dan
The output stage was notoriously under-biased, creating lots of crossover distortion at low levels. If you measured the distortion and didn't look at or think about the harmonics, the numbers seemed pretty good...in reality, it was pretty miserable classic notch (crossover) distortion...
Of course, if you let it warm up long enough, the bias current would creep up, and some of the worst distortion wouldn't be quite so bad...Perhaps this one amp is responsible for a world of audiophiles warming up equipment for hours before listening!
Then, there were a few out there, that probably sounded great, but the output stage bias was on the hairy edge...play them loud enough, long enough, and the magic musical smoke would escape from the amp as the quiescent current ran away...
Anyway, with this major shortcoming in mind (purists might point to a number of other shortcomings...), I designed an analog bias controller for the output stage quiescent current. It nails the output stage quiescent current wherever you'd like (I've settled on around 50 mA), and keeps it from varying over temperature, no matter how hard you beat on the amp.
You can kluge it onto existing working amps with very little fuss. The result is distortion behavior much like a tube amp...almost no distortion at low levels (e.g. unmeasurable harmonics with 10 mV rms input), and then as output amplitude picks up, you get more 2nd harmonic than 3rd. The distortion increases gradually until you approach clipping.
I figured I would just post this much to see if there's interest...if so, I can write up the circuit, and post some information...
All for now...comments invited...
Dan
Dynaco Stereo 120 Simulation Model in LTSpice
To establish a baseline, here is a simulation model for the Dynaco Stereo 120 amplifier channel. Play with it some...put in some small signals, 10 mV or so...look at the output spectrum...lots of harmonics, and not too far down...classic symptoms of not enough current in the output stage...look at the output stage currents...output transistors really aren't even conducting.
If you change one of the diodes (say D1) to a 1N5817, (a quick way to model a slight change to the output stage bias) see how much worse the distortion gets.
In an upcoming post, I'll add the bias controller, and show its effect.
The included files are:
1. stereo 120 standard schematic.jpg
2. dynaco stereo 120 stock with TIP Mod.txt - change the .txt to .asc and you can run it as a simulation file.
3. standard.bjt.txt - rename it to standard.bjt. After you have backed up your existing LTSPice standard.bjt file to another name, you can copy this new one in its place. It has all the models you'll need.
To establish a baseline, here is a simulation model for the Dynaco Stereo 120 amplifier channel. Play with it some...put in some small signals, 10 mV or so...look at the output spectrum...lots of harmonics, and not too far down...classic symptoms of not enough current in the output stage...look at the output stage currents...output transistors really aren't even conducting.
If you change one of the diodes (say D1) to a 1N5817, (a quick way to model a slight change to the output stage bias) see how much worse the distortion gets.
In an upcoming post, I'll add the bias controller, and show its effect.
The included files are:
1. stereo 120 standard schematic.jpg
2. dynaco stereo 120 stock with TIP Mod.txt - change the .txt to .asc and you can run it as a simulation file.
3. standard.bjt.txt - rename it to standard.bjt. After you have backed up your existing LTSPice standard.bjt file to another name, you can copy this new one in its place. It has all the models you'll need.
Attachments
Would your bias scheme work on, say, a Naim NAP140-type circuit?
Like the Dyna, the Naim is quasi-comp with NPN outputs, but it uses a dual-rail supply and requires no output cap.
Like the Dyna, the Naim is quasi-comp with NPN outputs, but it uses a dual-rail supply and requires no output cap.
I am not nearly smart enough to mess with simulation files but I do have a thought in the back of my mind to fool with early SS amps and see what the experience is like. I got a garage sale Mitsubishi discrete transistor preamp a few weeks ago and I am having fun with it.
B
B
On a Naim Nap140?
Something like it would probably work...It would require some adaptation, because the Dynaco original circuit was kind of "baroque"...If you can post a schematic, I can think about it a bit...
Something like it would probably work...It would require some adaptation, because the Dynaco original circuit was kind of "baroque"...If you can post a schematic, I can think about it a bit...
it would be relatively easy to upgrade the amp to a current feedback
amplifier with vasly improved performance..
here a link..
Simple 60 Watt Power Amplifier
use the base version, the only thing is to adapt a better bootstrap
than the one in this schematic, you just have to add a resistor
and a capacitor...you can also go complementary for the final stage
rather than quasi complementary...
i saw this schematic first in the 70s, i did built one at the time,
and it really sounded very good...
distorsion is about 0.03 at most in my current simulations...
regards,
wahab
amplifier with vasly improved performance..
here a link..
Simple 60 Watt Power Amplifier
use the base version, the only thing is to adapt a better bootstrap
than the one in this schematic, you just have to add a resistor
and a capacitor...you can also go complementary for the final stage
rather than quasi complementary...
i saw this schematic first in the 70s, i did built one at the time,
and it really sounded very good...
distorsion is about 0.03 at most in my current simulations...
regards,
wahab
Neat Link...
Wahab,
Thanks...that's an interesting link...I enjoyed reading it through...
It's interesting to note that the old Dynaco design is also current mode feedback.
Dan
Wahab,
Thanks...that's an interesting link...I enjoyed reading it through...
It's interesting to note that the old Dynaco design is also current mode feedback.
Dan
yes, this the reason i proposed this schematic which has
the advantage of DC coupled interstages...
you even wont have to drill the pcb, the current
layout being enough, just be careful in components connections...
regards,
wahab
Performance Under Bias Control
Here are two simulations of the amp: both are at 10 mV rms input at 2 kHz.
The first is stock, the second is bias control...harmonics drop from 46 dB below the fundamental to nearly 80 dB below the fundamental...actual measured performance is even better than the simulation!
Here are two simulations of the amp: both are at 10 mV rms input at 2 kHz.
The first is stock, the second is bias control...harmonics drop from 46 dB below the fundamental to nearly 80 dB below the fundamental...actual measured performance is even better than the simulation!
Attachments
Question, Is this the amplifier that had the output inductor wrapped around the electrolytic capacitor? I think I built about 3 of these...
Thanks for the memories....
Thanks for the memories....
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Last edited:
Yes it is...
Yes it is...the output inductors wound 'round the big blocking caps...The sight of a dirty old dynaco makes me smile...
Yes it is...the output inductors wound 'round the big blocking caps...The sight of a dirty old dynaco makes me smile...
Remember Keep the Kinks SOLID wire used in the construction... pre-built boards and detailed instructions.....LOL
Here's a typical Naim amp schematic. It's a pretty basic '70s quasi-comp RCA Handbook design with some refinements. Surprisingly they still make 'em like this.
An externally hosted image should be here but it was not working when we last tested it.
fwiw I've run 120 & 80 on K-horns and liked them as well or better than some more modern amps - 80 IIRC doesn't run nfb from the output cap and seemed a bit mushier
more than some modern amps...
Part of my like for the old Dynacos was their simplicity...the way they did so much with so little...It's almost tube-like in its simplicity...6 transistors...a bit more than the old tube amps...just 4 tubes...
Part of my like for the old Dynacos was their simplicity...the way they did so much with so little...It's almost tube-like in its simplicity...6 transistors...a bit more than the old tube amps...just 4 tubes...
Its not easy getting a good tracking bias circuit so what so people do is keep the bias right down. Some even dont put the bias transistor on the heatsink to compensate.
Bias Controller for a Standard Quasi Comp Amp
This is for Mr. dB...it's kind of different than the bias controller I've worked out for the stereo 120...in some ways, it's better...but it's so much more complex that no one in 1966 would have dreamed of doing this...Actually, it's based on a design by Evert Seevinck, just adapted a bit to fit on a standard quasi-comp amp. It could also go on a full comp amp. I have only done a basic tune-up, no extended tweaking, but its performance is quite good.
Eventually, I'll post the circuit for the Dynaco bias controller, but for now, this is kind of interesting...
There are two jpg's...the first shows the basic power amplifier. The second shows the bias controller...lots of transistors! but it works pretty well...it lets you set and maintain a pretty high bias current, which can do nice things for linearity...by servoing the bias, thermal runaway is eliminated...
(apologies...you might have to stare a bit to see how the two connect together)
If anyone's interested, I can post more info later...
This is for Mr. dB...it's kind of different than the bias controller I've worked out for the stereo 120...in some ways, it's better...but it's so much more complex that no one in 1966 would have dreamed of doing this...Actually, it's based on a design by Evert Seevinck, just adapted a bit to fit on a standard quasi-comp amp. It could also go on a full comp amp. I have only done a basic tune-up, no extended tweaking, but its performance is quite good.
Eventually, I'll post the circuit for the Dynaco bias controller, but for now, this is kind of interesting...
There are two jpg's...the first shows the basic power amplifier. The second shows the bias controller...lots of transistors! but it works pretty well...it lets you set and maintain a pretty high bias current, which can do nice things for linearity...by servoing the bias, thermal runaway is eliminated...
(apologies...you might have to stare a bit to see how the two connect together)
If anyone's interested, I can post more info later...
Attachments
Simple Bias Controller for Dynaco Stereo 120
In contrast to the previous post, I wanted to work out a bias controller that was simple enough to be conceivable in 1966. It may not have all the benefits of the previous post, but then it's a whole lot simpler. Please be advised that there is a patent pending on the circuit*.
You can build the bias controller on a small PCB, or bit of perf board. Q8 and Q9 should ideally be a matched pair. Q1, Q2, and Q10 don't need any matching.
Really short directions.
1. power down the amp.
2. remove D2, D3, and C6 from a PC14 board.
3. connect up the four interconnect wires as shown. Three are available at the edge of the board. Q5 Emitter _R27 top is available on the resistor as indicated.
4. Power up the amp. If you carefully measure the voltage across R27 with no signal, it should be pretty close to 20 mV. This represents about 40 mA of quiescent current.
Listen, measure, and enjoy.
*a copy or two built for your own enjoyment is fine. Copies made for profit or sale to others is not allowed.
In contrast to the previous post, I wanted to work out a bias controller that was simple enough to be conceivable in 1966. It may not have all the benefits of the previous post, but then it's a whole lot simpler. Please be advised that there is a patent pending on the circuit*.
You can build the bias controller on a small PCB, or bit of perf board. Q8 and Q9 should ideally be a matched pair. Q1, Q2, and Q10 don't need any matching.
Really short directions.
1. power down the amp.
2. remove D2, D3, and C6 from a PC14 board.
3. connect up the four interconnect wires as shown. Three are available at the edge of the board. Q5 Emitter _R27 top is available on the resistor as indicated.
4. Power up the amp. If you carefully measure the voltage across R27 with no signal, it should be pretty close to 20 mV. This represents about 40 mA of quiescent current.
Listen, measure, and enjoy.
*a copy or two built for your own enjoyment is fine. Copies made for profit or sale to others is not allowed.
Attachments
st120 user
Thanks for designing an update for this classic. I own a ST120, bought with one side output transistors blown, some resistors, power supply blown. Had no schematic, guessed on PNP/NPN of power supply. Bought some NEC's TO3's (181), replaced burnt parts, ran it a couple of months, smelled something, same channel had blown outputs and some discrete resistors. Didn't dislike the sound that much compared to my ST70 with the unobtainable parts (1985), main thing I didn't like was it was a lot louder than ST120 behind a PAS2 preamp, and had a faint frying bacon sound when no input. Repaired it again, replaced the electrolytics based on the date (1986) used it a year. Took it to church to amp a keyboard, at the end of a 3 hour chior rehearsal watched it go up in a 18" diameter ball of flame under the pulpit. Junior choir was impressed. Same 2 transistors and discrete resistors, had added a 1 ohm 10W power resistor from power supply, it went. Put it away, bought some TO3 NPN transistors, but gave up and went back to using my ST70. Been gathering heat sinks from motor drives and microwave oven themo-switches (to shut down if hot) I'm retired now, been psyching myself up to deal with it, especially as I have been listening to LP's in mono with the tube amp on my Hammond organ. Based on some recent reading, I believe the new TO3 transistors I bought had more hfe than the original 2n3055's on the other channel, too much bias leads to thermal runaway. I'm too dumb to spot the bias resistors on the Gregg Dunn schematic. My first thought after reading some stuff on music-electronics-forum.com was to increase the bias resistors values to compensate for the higher hfe modern 181 or 2n53xx transistors, but google spotted your article for me. Thanks. My power supply is still home brew by a linear idiot, I think I am going to put the aiken MOSfet regulator in if I can figure out how to buy the semi's from Newark. Uh, couldn't one use thermistors on the heat sink somewhere and cut the bias back if everything got hot? My first thought was to use the microwave temp switch to pickup a relay to pull a parallel resistor out from next to the regular bias resistor, but that might make a really bad transient in the speakers. My budget is really limited, I'm retired if I can live below the poverty level, but I can afford $100 of hobbies occasionally.
Thanks for designing an update for this classic. I own a ST120, bought with one side output transistors blown, some resistors, power supply blown. Had no schematic, guessed on PNP/NPN of power supply. Bought some NEC's TO3's (181), replaced burnt parts, ran it a couple of months, smelled something, same channel had blown outputs and some discrete resistors. Didn't dislike the sound that much compared to my ST70 with the unobtainable parts (1985), main thing I didn't like was it was a lot louder than ST120 behind a PAS2 preamp, and had a faint frying bacon sound when no input. Repaired it again, replaced the electrolytics based on the date (1986) used it a year. Took it to church to amp a keyboard, at the end of a 3 hour chior rehearsal watched it go up in a 18" diameter ball of flame under the pulpit. Junior choir was impressed. Same 2 transistors and discrete resistors, had added a 1 ohm 10W power resistor from power supply, it went. Put it away, bought some TO3 NPN transistors, but gave up and went back to using my ST70. Been gathering heat sinks from motor drives and microwave oven themo-switches (to shut down if hot) I'm retired now, been psyching myself up to deal with it, especially as I have been listening to LP's in mono with the tube amp on my Hammond organ. Based on some recent reading, I believe the new TO3 transistors I bought had more hfe than the original 2n3055's on the other channel, too much bias leads to thermal runaway. I'm too dumb to spot the bias resistors on the Gregg Dunn schematic. My first thought after reading some stuff on music-electronics-forum.com was to increase the bias resistors values to compensate for the higher hfe modern 181 or 2n53xx transistors, but google spotted your article for me. Thanks. My power supply is still home brew by a linear idiot, I think I am going to put the aiken MOSfet regulator in if I can figure out how to buy the semi's from Newark. Uh, couldn't one use thermistors on the heat sink somewhere and cut the bias back if everything got hot? My first thought was to use the microwave temp switch to pickup a relay to pull a parallel resistor out from next to the regular bias resistor, but that might make a really bad transient in the speakers. My budget is really limited, I'm retired if I can live below the poverty level, but I can afford $100 of hobbies occasionally.
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