I need to order a power transformer to an amplifier with the following specifications (per channel):
70-0-70V (AC), -94.5 - 0 - +94.5 v (DC)
Output RMS - 300 (8ohms), 400 (4ohms)
What power should I ask? Whereas will be 400 W rms into 4 ohms, 700VA per channel is enough?
I would correct the following specification?
Primary - 120v
Secondary - 2 x (70-0-70)
Power - 1400 VA
Any help is welcome ...
70-0-70V (AC), -94.5 - 0 - +94.5 v (DC)
Output RMS - 300 (8ohms), 400 (4ohms)
What power should I ask? Whereas will be 400 W rms into 4 ohms, 700VA per channel is enough?
I would correct the following specification?
Primary - 120v
Secondary - 2 x (70-0-70)
Power - 1400 VA
Any help is welcome ...
Thank you for the answer.
Yes, you are rigth, but it is an amplifier of the 70's - Marantz 2600.
70V AC is close to 99V DC (70 x 1.414 = 98 + 0.98)
The 94.5V DC spec may be the loaded secondary voltage.
300W continuous in 8 is 69V peak.
A power supply with 25V higher rail voltage is uncommon
(exception would be a Lateral MOSFET power amp with a soft power supply, relatively low value PS electrolytics, e.g. a Perreaux PMF-3150)
Aka, how many VA you need depends on what kind of power amplifier you have exactly.
Too many VA may kill your amp.
The 94.5V DC spec may be the loaded secondary voltage.
300W continuous in 8 is 69V peak.
A power supply with 25V higher rail voltage is uncommon
(exception would be a Lateral MOSFET power amp with a soft power supply, relatively low value PS electrolytics, e.g. a Perreaux PMF-3150)
Aka, how many VA you need depends on what kind of power amplifier you have exactly.
Too many VA may kill your amp.
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Yes, 94.5 is the loaded secondary voltage.
Below, some photos of the amplifier and the wiring diagram. I have an original transformer working and need another transformer to another amplifier.
Attachments
The output stage has plenty of power transistors for 300W/8 (originals are B600/D555, 8 x 200W is 1600W Pd total).
But they're mounted inside a fan blown air duct, no telling what effectively remains of those 1600W.
The 2600 has wimpy sized electrolytic cans for a 300W amp, means the rail voltage will drop heavily into a low impedance load.
Hence the 400W/4 spec.
Easiest would be to weight the original PS toroid, it will tell you the VA.
But they're mounted inside a fan blown air duct, no telling what effectively remains of those 1600W.
The 2600 has wimpy sized electrolytic cans for a 300W amp, means the rail voltage will drop heavily into a low impedance load.
Hence the 400W/4 spec.
Easiest would be to weight the original PS toroid, it will tell you the VA.
That´s new for me. I will weigh the transformer and back with the information.
For completeness, a second reason to be careful with a toroidal transformer swap is the exchange to MJ15024/5.
OnSemi TO-3s have a higher hfe, but it drops considerably at high output current levels.
The original NEC devices have a modest hfe spec value, but their hfe curve remains much flatter.
Means that at high output levels, the drivers may need to deliver more current.
The B630/D610 drivers are fed from the same rails as the output devices.
Picking a replacement transformer of a much higher VA value will lead to stiffer rails, also for the driver pair.
Higher current plus higher rail voltage level is much higher dissipation for the drivers, they may blow before the output devices do)
In particular, as the drivers are also mounted in the heat duct.
Fans on heatsinks are like a turbo-charger on a car engine : more power, but the response is slower.
(a single pair of 25W drivers, on 95Vdc rails, is also an indication that the transformer VA will be at a modest level)
OnSemi TO-3s have a higher hfe, but it drops considerably at high output current levels.
The original NEC devices have a modest hfe spec value, but their hfe curve remains much flatter.
Means that at high output levels, the drivers may need to deliver more current.
The B630/D610 drivers are fed from the same rails as the output devices.
Picking a replacement transformer of a much higher VA value will lead to stiffer rails, also for the driver pair.
Higher current plus higher rail voltage level is much higher dissipation for the drivers, they may blow before the output devices do)
In particular, as the drivers are also mounted in the heat duct.
Fans on heatsinks are like a turbo-charger on a car engine : more power, but the response is slower.
(a single pair of 25W drivers, on 95Vdc rails, is also an indication that the transformer VA will be at a modest level)
Hi jacco
Thanks for the good information in your post. The weight is 8.5kg. The first picture is of the transformer being weighed.
A few years ago, I was working with a Marantz 2600 with the burnt transformer. So I had to make some assumptions about the voltages, the results are the following links:
Now own the original transformer and the previously estimated voltages were confirmed. But not sure about VA for power amplifiers. This is now my problem ...
Anyway, I was thinking of redesigning the circuit in a new layout and see what can be achieved with this circuit, using new components. It will be a challenge, but for a hobbyst, every challenge is welcome.
Thanks for the good information in your post. The weight is 8.5kg. The first picture is of the transformer being weighed.
A few years ago, I was working with a Marantz 2600 with the burnt transformer. So I had to make some assumptions about the voltages, the results are the following links:
- Very interesting info on the 2500 and 2600 tranny - AudioKarma.org Home Audio Stereo Discussion Forums
- http://vintagesilversmith.com/Marantz/2500_2600_Toroids.pdf
Now own the original transformer and the previously estimated voltages were confirmed.
But not sure about VA for power amplifiers. This is now my problem ...Anyway, I was thinking of redesigning the circuit in a new layout and see what can be achieved with this circuit, using new components. It will be a challenge, but for a hobbyst, every challenge is welcome.
Attachments
You'd have to subtract the weight of the (aluminum ?) can and the potting compound off the 8.5kg.
A 20cm diameter and 10cm high aluminum can of 1mm thickness weighs 1/4 kg.
Potting compound does 1.2-1.4 kg per liter.
Suppose the total is 1kg, leaves 7.5kg for the toroid, ~16lb.
Large toroidal transformers do 1.5-2 lb/100VA
Means your toroid will be in the range of 800-1000VA
(type toroid + weight in google for example tables/graphs)
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Hi Andrew
My goal in this thread is to specify more adequately the toroidal transformer to the amplifier section.
Whereas the voltages were confirmed, which really lack is the specification of power in VA of the transformer.
I have three Marantz 2600. The first one had a burned transformer. I requested a new, custom made, transformer of a 700VA. It is working perfectly. The second is transformerless and I will order a new one. Here is the question? 700VA is suitable? The output devices were changed. The third marantz has the transformer running smoothly.
Ha, do not ask me because I have 3 Marantz 2600. I do not have that answer ....
:
In the solid state amp section I will open a thread for a second goal: to clone a Marantz 2600, considering only the amp section. The specification is very good: Input sensitivity 1.5V, output power 300w 8 ohms. Just as an exercise, I will try to clone the amplifier building a new layout, using current devices, and I believe that with 4 pairs of MJ can go far beyond 300w.
See, it's not a necessity, but a way to play and learn ...
My goal in this thread is to specify more adequately the toroidal transformer to the amplifier section.
Whereas the voltages were confirmed, which really lack is the specification of power in VA of the transformer.
I have three Marantz 2600. The first one had a burned transformer. I requested a new, custom made, transformer of a 700VA. It is working perfectly. The second is transformerless and I will order a new one. Here is the question? 700VA is suitable? The output devices were changed. The third marantz has the transformer running smoothly.
Ha, do not ask me because I have 3 Marantz 2600. I do not have that answer ....
:In the solid state amp section I will open a thread for a second goal: to clone a Marantz 2600, considering only the amp section. The specification is very good: Input sensitivity 1.5V, output power 300w 8 ohms. Just as an exercise, I will try to clone the amplifier building a new layout, using current devices, and I believe that with 4 pairs of MJ can go far beyond 300w.
See, it's not a necessity, but a way to play and learn ...
Try to identify which parts habitually fail and why.
Then find a way to prevent that happening again.
This may mean a smaller transformer to put the output stage under less stress and so survive longer.
That enormous voltage transformer is probably the part most likely to be overstressing the amplifier.
Then find a way to prevent that happening again.
This may mean a smaller transformer to put the output stage under less stress and so survive longer.
That enormous voltage transformer is probably the part most likely to be overstressing the amplifier.
I believe that combining the high voltage with reduced space for air exchange is the weak point of this project.
Thus, a new version of this amplifier, with new devices/components and a new layout can lead to very satisfactory results ....
Just an exchange of the original heatsink duct for a proper one would be a big help.
For example a modular one as in the old Krell amps, see the KSA50/100 threads for images. Complete fan/heatsink units often go for reduced prices.
Currently available fan types are way less noisy, and lots cheaper.
For example a modular one as in the old Krell amps, see the KSA50/100 threads for images. Complete fan/heatsink units often go for reduced prices.
Currently available fan types are way less noisy, and lots cheaper.
Hi Jacco,
I followed your advice and I did a search on google. The first one about the weight of transformers and the second about the KSA 100. Thank you for the advices.
The tables that I found really help a first approach to the power transformers. I will go with 600VA/ch for this project.
What about the sinks I found for some KSA 100 clones are exactly what I want to clone Marantz 2600. But it will take some time until I finish to design this new layout ... I need, at least, finish this revamp...
I followed your advice and I did a search on google. The first one about the weight of transformers and the second about the KSA 100. Thank you for the advices.
The tables that I found really help a first approach to the power transformers. I will go with 600VA/ch for this project.
What about the sinks I found for some KSA 100 clones are exactly what I want to clone Marantz 2600. But it will take some time until I finish to design this new layout ... I need, at least, finish this revamp...
How nice to read something in Portuguese ....
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