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Old 17th November 2004, 10:17 AM   #1
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Talking OTL PS question!!

Iím going to build the famous 6C33 OTL design by Mr. Ciuffoli and Iím planning to use a regulated HT for both the drivers and the final stages. So if you notice on the schematic the 250VAC from the transformer goes to the rectifier bridge and we have about 352V DC. Now we have two possibilities Circuit 1 or 2. Which one you think is the best to use? Please note that I already have in my hands these 6000uF capacitors. Also in the next message I will send the regulator circuit Positive and Negative with the discrete parts.

Thanks in advance
Chris
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Old 17th November 2004, 10:26 AM   #2
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And here are the regulators (positive and negatine)
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Old 17th November 2004, 11:02 AM   #3
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#1, definately, with a centre-tapped tranny.

6,000uF?

Wouldn't that cause excessive transformer heating and diode spikes? Do bridge your diodes with snubber caps (0.01 @ 1KV disk) to keep the transient noise down
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Old 17th November 2004, 11:31 AM   #4
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Hi,

Maybe not the answer you are looking for but I suggest that you build the OTL amp without regulators, it works well if you use good components, (especially the PS caps). In my OTL that is similar to the Ciuffoli circuit I use 2 x 5500uF as only filtering for the power tube supply, it works very well. For B+ to the driver I use CRC with 100uF + 100uF and then one RC stage with 220uF to feed the input stage, (SRPP).

Regards Hans
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Old 17th November 2004, 12:28 PM   #5
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Hans,

I was thinking of using the regulated PS because I will use this amp with a set of back loaded horns using FE208ES units with total sensitivity of about 100dB, so as you can see I dont like to have any irritating HUMMM on the speakers. Also I think that in case I will use this PS, there is no need of any chokes because the ripple of the voltage will be very very small, finally the cost and the weight of the chokes also is importand.
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Old 17th November 2004, 07:31 PM   #6
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....Blink....

For one, if you have a regulator, why the heck are you worried about hum anyway?

A bad regulator anyway as MOSFETs are transconductance devices and they will allow a few volts of droop.

Tim
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Old 17th November 2004, 09:25 PM   #7
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Quote:
so as you can see I dont like to have any irritating HUMMM on the speakers.
Yes I can appreciate that as I am using Lowther horns myself. My amp gives 0.4mV Pk-Pk noise and hum which gives signal to noise ratio of >100dB, this without using any chokes and no regulators only caps, I can not hear any disturbing hum at all using my Lowther Fidelio horns which have about ~100dB sensitivity so I think it is quiet enough.

Regards Hans
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Old 17th November 2004, 09:34 PM   #8
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Default Regulation or no regulation ....

Just to cast my vote here, my impression, from direct experience and otherwise, is that power supply regulation benefits the overall sound and fidelity of the amplifier stage in question, including that of the output stage. Regulation can be complicated, surely, but so are many a beneficial thing such as, say, women, which can be seen to benefit life.
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Old 17th November 2004, 10:03 PM   #9
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Hi,

Quote:
power supply regulation benefits the overall sound and fidelity of the amplifier stage in question, including that of the output stage.
Generally speaking I'd agree but let's look at the facts here:

This OTL runs class AB1.
Max power into 8 Ohm load is about 25W tops for this design.

Speaker has ~ 100 db/W/m effeciency.

This means that for all intents and purposes the amp is likely to spend most of its life outputting 1 to 10W at most.

Regulating the B+/- of an OTL amp isn't simple and as far as hum is concerned, most of it can effectively be taken care of with decent filtering and careful layout.
Moreover, this is a PP amp so a huge chunk of the hum will effectively be cancelled out anyway.

In practice, a good OTL amp can be as quiet as any other topology.
In fact, I often find them quieter than most others, perhaps because more attention is paid to the design and layout. Who knows.

As for choke filtering the main PS, great idea but where on earth are you going to find such animal?

The Ciuffoli design can benefit from a regulated driver PS IMO but it will add to TOC.
It would however be a great idea to study the circuit and see how we could improve on it by sticking our heads together...

Regarding the PS cap bank; my own OTL uses two banks of 2*10.000 ĶF/200V. I suppose Mr. Joule would be proud of me...


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Old 17th November 2004, 10:44 PM   #10
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Hi,

Regarding regulators I don't think they are trivial to design as to not influence the sound as the power supply can be seen as being more or less, (depending on the PSRR) part of the signal path, therefore I try to avoid them as much as possible. Anyway I don't go trough the extra work to use tubes just in order to listen to a solid state regulated power supply.

In my own setup I only use regulators, (tube based) in my preamp as I believe this is an efficient way to get low hum and noise levels, a preamp is a class A amp with normally decent PSRR so I think a regulator can be accepted there.

Anyway for an OTL amp of the type discussed here there is no need to use regulators in order to get it quiet, what is needed is some good PS caps, a good physical layout and some thinking when doing the wiring, especially the grounding.

Regards Hans
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