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Old 23rd March 2014, 04:40 PM   #1591
biju55 is offline biju55  India
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Nice build , what is the power out put of this amplifier in class A .
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Old 23rd March 2014, 08:18 PM   #1592
6L6 is offline 6L6  United States
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Quote:
Originally Posted by bcmbob View Post
. Looks like you took full advantage of the many ground pads on the PS board for the 'star'. Is the green wire from the AC entry module connected back to the PS? Can't see a termination there.
The AC safety earth is brought to the chassis. (it's actually not connected in the photos...)

Yes, the abundance of pads on the PSU ground was used to good effect, the amp is very quiet. The green from the FE board is connected to the PSU as you suspect.

Quote:
I'd be interested in hearing about the adjustment process/sequence and values you used - but that may be something included in the upcoming formal build guide. As the earlier posts show I struggled a bit there, but through that process now have a better understanding of the interdependence. Most likely less intimidating than it first appeared.
Basically I built and tested the FE as a seperate unit, set it's operating bias and offset, and them built the rest of the amp. Bias and offset on the output stage are very straightforward, it's essentially an F4, (this is a complimentary, not SE, output stage) so you merely need turn the pots to adjust them, there is no "see-saw" like an F5.

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SQ wise, have you had time to try any adjustments of P-3. I understand that is basically a user preference area, but I'm aware you have the lab tools to make some links between the art and the science there.
I haven't messed with it that much, but I have dialed P3 for minimum distortion. This nulls out a bunch of the 2nd harmonic, making it very, very clean and detailed, at the expense of warmth. I put it back to neutral, which is 2nd harmonic (slightly) dominant.

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Lastly, what are the specs on the transformer, and can you give some basic info in the speakers you are driving?
Plitron 18V+18V 400VA. Used as it was on hand. Speakers I have heard it on are Zaph audio ZA5.2. Zaph|Audio - ZA5 Speaker Designs with ZA14W08 woofer and Vifa DQ25SC16-04 tweeter This amp sounds much better than the F5T with these speakers.

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Originally Posted by Ryssen View Post
Nice! Are those heatsinks the "biggest" one from conradheatsinks?
No, they are Hifi2000/Modushop 300mm deep, the chassis is the diyAudio 4U deluxe.

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Originally Posted by mstomaso View Post
Very nice. Bias it hot and play it loud!
0.5V measured across the 1ohm source resistors. Approx. 11W/device and 135W total bias.

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Originally Posted by biju55 View Post
Nice build , what is the power out put of this amplifier in class A .
This has a 18v+18V transformer making 23.5V rails. I measured 35W into 4ohm, I'm sure slightly higher figures, perhaps up to 50WPC/4ohm could be attained with smaller source resistors.

Increase the rail voltage and the power will easily go up. The one built for the build guide will have 32V rails.

Last edited by 6L6; 23rd March 2014 at 08:20 PM.
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Old 23rd March 2014, 08:39 PM   #1593
bcmbob is offline bcmbob  United States
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Thanks,
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Old 25th March 2014, 10:46 AM   #1594
giuliodido is offline giuliodido  Italy
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Quote:
Originally Posted by 6L6 View Post
0.5V measured across the 1ohm source resistors. Approx. 11W/device and 135W total bias.

This has a 18v+18V transformer making 23.5V rails. I measured 35W into 4ohm, I'm sure slightly higher figures, perhaps up to 50WPC/4ohm could be attained with smaller source resistors.

Increase the rail voltage and the power will easily go up. The one built for the build guide will have 32V rails.
Interesting! From the F5T article I read that decreasing (or removing) source resistor will deliver, for a given bias figure, more power into class A. This also brings to a thermally instable circuit.

What is the soft spot for this trick? Is it somewhat related to the number of output devices used? I'm asking because for the 4-6 deep complimentary are used 1 Ohm resistors, while for a more simple 2 deep complimentary, Nelson suggests 0.33 Ohm resistors.
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Old 25th March 2014, 10:58 AM   #1595
AndrewT is offline AndrewT  Scotland
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The closer you match the transistors to lower you can take the source resistor as far as matching the quiescent current.

But Output stage temperature stability is also controlled by the Source resistor value.
I'd guess that 0r33 is a safe value for well matched FETs.
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Old 25th March 2014, 11:36 AM   #1596
bcmbob is offline bcmbob  United States
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It would seem three should be a formula with three elements: source voltage, matched transistor value and resistor value = watts/BTU. Anyone aware of such? I'm interested in lowering the resistor value on my build but hope there is a batter method than simple trial and error.
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Old 25th March 2014, 12:38 PM   #1597
6L6 is offline 6L6  United States
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Just use 0.47, Nelson uses them everywhere else... The 1ohm were used as the original design had 6 (or more) pairs/channel.
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Old 25th March 2014, 12:56 PM   #1598
bcmbob is offline bcmbob  United States
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Great - work on the complementary outputs should start shortly - I'll use those.
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Old 25th March 2014, 01:11 PM   #1599
6L6 is offline 6L6  United States
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Before you get too deep into it, I would suggest changing 2 values on the bias board - R125 to 13K, and P102 to 2K. That will give you more swing in the circuit to zero the offset.
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Old 25th March 2014, 01:29 PM   #1600
bcmbob is offline bcmbob  United States
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Are you going to build 'standard' or 'HOT ' for the illustrated guide?

I have no problem waiting for your lead at that time. The new amp will also have two output boards per channel - if that need to be considered.
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