Hello, maybe question can be to what point to increase power supply voltage, of course completely changing transistors output stage(MJ15024,15022,...), preserving quality and increasing output power?Yes, it is similar to Bose 1801 but is not same,..
Schematics is suggested design from old RCA power transistors datasheet book.
Waiting for interesting comments,...
Schematics is suggested design from old RCA power transistors datasheet book.
Waiting for interesting comments,...
Similar to Bose 1801, similar to BGW500/750. You can't increase the supply voltage by much, even with more modern devices. They all run up against the same limitations at elevated voltage. +/-85 or so is about as high as you can go, highest I've seen successfully is +/-93 (QSC USA1310) with MJ15022/4's.
The same kind of front end can be put on a S-Leach cascode OPS. I've run this off +/-127V with two lab horns in parallel (about 1.5 ohms). Other than a ridiculous AC current draw, it runs fine.
The same kind of front end can be put on a S-Leach cascode OPS. I've run this off +/-127V with two lab horns in parallel (about 1.5 ohms). Other than a ridiculous AC current draw, it runs fine.
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Thanks wg_ski for suggestions, i think this RCA schematics can without problems run at +/-100V with MJ15024, only i dont know what output power it will be on 8ohm due to emmiter resitor value1ohm transistor working point decision,..
About 350 watts at 8 ohms. Careful with 4 ohms, and don't run 2 ohms at those VCE's.
You don't need 1 ohm emitter resistors with modern transistors. 0.5 or even 0.33 is fine if you use transistors with the same batch code and made after 1990.
You don't need 1 ohm emitter resistors with modern transistors. 0.5 or even 0.33 is fine if you use transistors with the same batch code and made after 1990.
If you could keep the supply rails fixed at 100V, sure. The USA1310 produces 325W at 8 ohms from +/-93 unloaded, reducing to +/-78. Claims 400W, but that's at 1% distortion, which is clipping. The old PL700 produces 380W unclipped from +/-110V until it explodes 🙂. Most practical power supplies just aren't much stiffer.
The 1700 is the same as the USA1310. Minor schematic updates and a change from Toshiba outputs to Motorola - but it's the same power supply.
Looking at the datasheet for RCA1B09 (R.C.A Solid State Power Devices 1978) I see that increasing the supply voltage from around 86 to 100 halves the current that can be safely conducted. That implies doubling the number of output transistors for this particular device.
If you are going to use ON Semi equivalent parts the number required would have to be capable of handling the same current as the 16 R.C.A power transistors needed in each supply rail. You would need to double the current handling ability of the driver stage as well.
There is a matching safe operating area protection circuit. This is scaled to work with 1R emitter resistors and any reduction in that value will marginalize the safe operating area of the output stage in proportion. Extra devices will also have temperature implications - there is a temperature cutout device and if you are to make use of the hoped for extra power you would not want this cutting in before the power potential is reached.
Increasing rail voltages is not something to be undertaken lightly. If you get it wrong odds are that transistors will blow or short out before fuses.
If you are going to use ON Semi equivalent parts the number required would have to be capable of handling the same current as the 16 R.C.A power transistors needed in each supply rail. You would need to double the current handling ability of the driver stage as well.
There is a matching safe operating area protection circuit. This is scaled to work with 1R emitter resistors and any reduction in that value will marginalize the safe operating area of the output stage in proportion. Extra devices will also have temperature implications - there is a temperature cutout device and if you are to make use of the hoped for extra power you would not want this cutting in before the power potential is reached.
Increasing rail voltages is not something to be undertaken lightly. If you get it wrong odds are that transistors will blow or short out before fuses.
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Of course, changing 1R emitter, to R39 causes different protection result, must also be calculated to lower resistance of protection circuit input. When increasing power supply voltage is in project then i compare Phase Linear 700 series II with these RCA one output stage, diy PL700 with 10xMJ15024 chanell and voltage +/-110v capable to 500W RMS/8ohm-approved, and this RCA with 16xMJ15024 with same voltage +/-110V must be capable to run 1000W RMS/4ohm(8ohm?-maybe 550W) Also diy copy of phase linear 700 series two output stage operates on tunnel fan heatsink, without that- thermal overload,..
If you reduce the emitter values to 0R39 you might consider installing low value resistors in the base feed to each output transistor so the output currents are evenly shared. With 16 output devices there is a greater chance of encountering one which for some reason will have more current gain than the others and hog more current leading to failure. Temperature effects increase current gain and with a fan blowing to the opposite end in a tunnel you might consider accumulation of heat in the airstream down the chain of devices.
There may be good reasons why Amcron make bridge amplifiers?
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I downloaded the Service Manual for the PL700 - I had a feeling this might operate in Class B mode rather than Class AB as per the R.C.A. circuit.
Considering the original R.C.A. with 1R emitters, a voltage drop of 50 m.v. across each, would result in Iq (quiescent current) of 400 m.a. It would be a good idea to reduce the emitter voltage drop to 25 m.v. if you were to double up on the output device numbers.
In the PL700 the voltage drop across the bases of the output transistors is specified to be .2 volts with a maximum of .6 volts (R109 refers). The median of 0.4 volts would put this in the realm of Class B where the unit would run cooler.
The Service Manual was interesting to read.
Yes, Phase Linear described very well service tips and checking procedure.
i Will not touch 1R emitter resistors, they are not projected by stupid engineer,...
"MJL4281A"
Half the SOA of the MJ21195/96 at 100V and a Tj of only 150°C vs 200°C.
Assuming an amplifier operating at ±100v, with a junction temperature of 100°C, you would need four of the plastic devices to equal one of the metal.
Half the SOA of the MJ21195/96 at 100V and a Tj of only 150°C vs 200°C.
Assuming an amplifier operating at ±100v, with a junction temperature of 100°C, you would need four of the plastic devices to equal one of the metal.
QC triples often have stability issues. Run one on +/30V, and it is stable with absolutely ANY bias current. From full class B to full class A and everywhere in between. Crank it up to the +/-80V range and everything changes. They are stable in class B (aka PL700) and at high bias (like the RCA circuit and its variants). But try to bias the output stage at something sensible - like 3 to 5 mA per devcie (which would result in a class AB with reasonable idle dissipation) and they break into oscillations on the negative half cycle. Sometimes destructive, but not always. But always VERY annoying and usually hard to get rid of any other way than changing the bias. Use a full complementary, which is possible with moden MJ2119x devcies and you can pretty much bias it anywhere. With mine I usually run around 3-5mA per devcie, which would normally be considered "underbiased", but with as much loop gain as these topologies have at 20 KHz crossover distortion isn't an issue. Obvioulsy so, because you CAN run them class B.
I am interested in these observations and have tried to understand possible causes.
From reading the service manual, the design intention appears to be that the bias current (Iq or standing current) is provided by the driver stage. R109 is a 10R resistor between base and emitter of the output transistors - the drop is set to be between 0.2 volts minimum and 0.6 volts maximum.
In each amplifier output half that would imply a driver stage bias current to the speaker, under quiescent conditions, of between 20 m.a. and 60 m.a. which ought to be a reasonable enough Class A cushion against crossover distortion.
In terms of device transfer slopes it seems to be a reasonable idea to keep these as clean and simple as possible within the cross over region - and have a wider non conduction zone for the output stage if it is to be comprised of 5 transistors with albeit slight variations among them.
As I see it a bias current between 3 m.a. and 5 m.a. per output device risks multiple turn on and turn off encroachments into the Class A cross-over region - this reverses normal situation since the driver stage has the dominant current in this domain. Trumping that would require biasing the output stage well into Class AB.
The issue of oscillation is covered in the Service Manual replacing the driver transistors is one of the solutions. I imagine these have a pretty hard life without increasing the current to provide a Class AB output stage.
I remember reading somewhere, an advertisement I think, where the PL700 was claimed to operate in true Class B. I would follow the makers recommendations.
I note the output devices 66546 are RCA410's relabeled -these heavy duty but low fT of 2.5MHz. I notice your choice of complementary replacement devices have fT values of 4MHz which would no doubt enhance loop stability. I think NPN versions would do the same in a quasi complementary amplifier.
By way of comparison those in the RCA amplifier have fT values of 5MHz.
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