Higher Voltage Transformers

David Ellis · « **on:** 18 Apr 2005, 04:50 pm »

On Rod Elliots page there is some discussion concerning the use of slightly higher voltage transformers for use in his amplifiers when driving higher impedance load loudspeakers. Is this same truth possible with the AKSA 55?

The low DCR of my speakers is 6.2ohms. Could I use a higher votage transformer with the AKSA 55 for driving this load and still avoid smoke? If I can do this, how high could I go with the voltage?

Also, if I smoke a few parts during the experiment, that's okay. This is a learning experience.

I certainly understand this relationship might only hold true for Rod's amplifier projects.

Thanks in advance.

andyr · « **Reply #1 on:** 18 Apr 2005, 10:23 pm »

Quote from: David Ellis

On Rod Elliott's page there is some discussion concerning the use of slightly higher voltage transformers for use in his amplifiers when driving higher impedance load loudspeakers. ...

David,

Could you define whether Rod is talking about using a slightly higher secondary voltage on the power transformer ===> higher +/- DC rails which I would've thought would be a baaad thing (I'm sure Hugh will comment IDC) or is he talking about an "autotransformer" which sits between the amp and the speaker, and changes the DCR which the amp sees?

Regards,

Andy

David Ellis · « **Reply #2 on:** 18 Apr 2005, 10:35 pm »

I am addressing the secondary winding voltage in the amplifier power supply. I am not addressing a loudspeaker impedance matching transformer.

Rod Elliot addresses this with his amplifier project here: http://sound.westhost.com/project3a.htm

AKSA · « **Reply #3 on:** 18 Apr 2005, 10:56 pm »

David,

The AKSA 55W is tolerant of higher rail voltages to 42V. Beyond this, certain complex speaker loads can take the single pair output stage out of the SOAR parameters, blowing the devices.

The 100W AKSA, a very similar design, uses two output pairs and is safe to 52V rails. A four output pair AKSA has been built by a number of AKSAphiles, most notably Romeo in Brindisi, but the rail voltage is the same; the purpose of the larger output stage was to dump more current into very low impedance speakers, as most high end larger speakers dip below 4R. These larger amps are problematic because of the very large power toroids, which should be fitted with slow start circuits.

A 42V rail 55W will require just one part change, the LTP feed resistor, but will not sound as 'big' as the 100W AKSA at the same voltage and with the same source/speaker. There is a palpable sonic difference. It will however generate around 80W into 8R, and about 120W into 4R, depending on the regulation of the power transformers. With the 4R load there is ever-present danger of breaking the output devices with impedance dips, and this is the reason I specify 36V rails for the 55W. Reliability has never been an issue, and power is more than sufficient.

Cheers,

Hugh

David Ellis · « **Reply #4 on:** 19 Apr 2005, 12:46 am »

Most excellent Hugh! I kinda figured this would be the answer. I'll be having the transformers done from these folks: http://www.tortran.com/

Given a 25-0-25 what VA rating to you recommend? I read the material on your site, but it never really conveys the most ideal VA rating.

Given a 30-0-30 what VA rating do you recommend?

Thanks Hugh,

Dave

AKSA · « **Reply #5 on:** 19 Apr 2005, 02:30 am »

Hi David,

For a 30-0-30Vac toroid, I recommend somewhere between 180VA and 225VA for each channel.

But, there are problems here for Tortran. They only make larger toroids, starting at 400VA, for the 30-0-30Vac components. I've check both single and dual primaries, no dice. Highest they go for 180VA/200VA is 2 x 22V, which is a bummer.....

You would be better advised to try this toroid from http://www.Plitron.com:

067017201 2x30Fac 3.75A per winding

or the 216AD_D series from http://www.bcrn.com

You must have the right voltage, of course, but you can futz a little with the power rating.

Hope this helps, your package went out this morning!

Cheers,

Hugh

Occam · « **Reply #6 on:** 19 Apr 2005, 12:42 pm »

Dave,

I don't know if this is relevant to your situation in choosing a secondary voltage for your transformers. Here in the Nasty Apple my line voltage runs about 126VAC throughout the year which is about 10% over the nominally specified standard of 115vac. (some manufacturers specify their primaries @ 115VAC, some @ 120VAC) I found it necessary to linerarly adjust my secondary voltage to reflect both the quoted primary voltage and the actual line voltage. It worked out well as I was able to trade my 35v secondary 330va trans for my 100s for 30v secondary trans with someone who had consistently low line voltages.
FWIW

netaron · « **Reply #7 on:** 21 Apr 2005, 03:55 am »

Dave,
I built an MTM version of the same drivers used in your 1801's, and mine are connected in parallel which takes the impedance down to 4ohms, I also built the 55N+ and so far I have not experienced any problems using the recommended transformers and the amp sounds wonderful.

Haron

David Ellis · « **Reply #8 on:** 23 Apr 2005, 12:07 am »

I appreciate you input and ordered transformers that will give me 35 volt rails and another pair that will give me 42 volt rails. I look forward to this experiment.

I really apprecaited the comments about the higher voltage levels in the Northeast. I guess this is pretty common in that entire region.

Tortran was able to provide the transformers. I should have them in 3 weeks.

Steven Harrison's AKSA 55wpc amplifier's continue to sound very good, but... I'll have to return them pretty soon.

Dave

AKSA · « **Reply #9 on:** 23 Apr 2005, 12:46 am »

David,

I'm glad Tortran were able to come to the party.

As a general rule, with global negative feedback amplifiers, meeting the Bode/Nyquist criteria for stability becomes much more difficult with increasing rail voltage. This is because the stability margin is tight to achieve the nimble handling characteristics of high quality amplifiers, and raising rail voltage threatens this situation. By rough analogy, and most apt in your profession, you can compare the stability equation to the difference between a jet fighter like an F4 versus a transport airplane like a C130; the former is poised on the edge of dynamic instability as a necessary condition of high manoeuverability. You can stretch the envelope evenfurther on a sophisticated fighter (or the Space Shuttle) by interposing a computer between the pilot and the control surfaces; I've used a few circuit tricks on the AKSA to achieve the same thing. The AKSA is very tight, yet it is extremely stable while showing incredibly nimble 'handling'.

I will be driving over to see Steve Harrison in Ballarat, country Victoria, on Tuesday. I will pass your regards to him!

Cheers,

Hugh