Too much voltage

[bushroot]

I'm building an ESP P101. The transformer seems to have a higher output voltage than what I need. I was expecting +-70VDC. Instead, I have +-80. I was curious what I could do to reduce the output voltage of the power supply without resorting to unwinding the secondaries. Could I possibly use a larger resistor value in the RC network?

[robbie b]

Dear Bushroot,

if you are using a 50 volts transformer than you will have a 70 volts UNDER NORMAL LOAD.
If you don't have a load and just measure the voltage than it can be signifintely higher. Especially when one is using a small transformer, below 50-80 VA. The lower the VA's the bigger the difference between the voltage with and without load.
Rule of thumb is that the voltage comming of a rectifier is about 1,4 times the ac voltage, with full wave rectifiing.

rob

[richie00boy]

Yes you could use a larger resistor in the CRC power supply but it wouldn't make much difference at small currents so you really need to make sure your transistors are rated at least 160V. You will get away with it if you stick with an 8 ohm load or higher.

Of course you are measuring the voltage with full load, aren't you? If not then it will be higher at low load conditions, maybe upto 10% higher.

[bushroot]

I'll play around a bit with the rc network. The mosfets I'm using are rated for 160v, so I'm not quite as concerned as I was. I guess I'll pull the amplifier off the variac and see how things go.

Thanks,
Christopher

[Eva]

Additional turns may be added to the transformer without rewinding it and connected in series with the primary or the secondaries

Depending on the chosen polatiry, this may increase or reduce the output voltage. I think the easier approach is to add turns to the primary and connect them in phase [so the output voltage is reduced]

[bushroot]

I contacted Rod. He says that he's tested the amplifier to this voltage and that it should be alright. I'm going to power it up without the variac on the mains and see if I let the smoke out.

Thanks everyone,
Christopher

[AndrewT]

Hi,
CRC PSU could be changed to RCRC using about 1ohm for the resistors. A hi bias in the output stage will pull the voltage down quite significantly. The first C could be much smaller than the final C and help reduce the current pulse at switch on.
Try thinking about a capacitance multiplier, there's a live thread just now.

[K-amps]

Originally posted by bushroot
I contacted Rod. He says that he's tested the amplifier to this voltage and that it should be alright. I'm going to power it up without the variac on the mains and see if I let the smoke out.

Thanks everyone,
Christopher

Don't you guys use lamps on the trafo's primaries? No wonder you guys fear "smoke".... with a lamp, you dont have to see smoke again...

PS: With 80v rails (assuning the amp survives) you get more power.... but I am not telling Rod if you dont.

[bushroot]

Well, it lives. I don't have time to take pictures of it right now. I can't believe how loud ~180W/channel is! I was previously using a Mitsubishi DA110 (110W/channel) and the difference is incredible. I'm using a power supply similar to that shown here:
http://www.tnt-audio.com/clinica/ssps2_e.html I've changed it somewhat though. I have a single 800VA toroid feeding 2 full wave bridge rectifiers. The first stage of filtering is 4 7700uf computer grade caps (3X6") with 20W 1ohm resistors between them. The second stage is 4 10000uf snap in caps, followed by a third stage consisting of 4 150uf snap in caps. After that, I used 2 500uf caps (non electrolytic) in series with another set of 20W 1ohm resistors. I think I've been bitten by the bug. Next on my list is a DIY preamp using these controls: Remote control kits

Thanks everyone. I was a bit nervous having spent over 2 months building this thing.

Regards,
Christopher

[richie00boy]

Originally posted by K-amps
PS: With 80v rails (assuning the amp survives) you get more power.... but I am not telling Rod if you dont.

We know you get more power, but the issue is that there is a very real danger of encroaching on the output devices SOA. For any given load the danger becomes worse at higher operating voltages as you move up the SOA curve, even if you only let it play to the same power.