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Old 22nd November 2008, 07:00 AM   #31
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Quote:
Originally posted by androuski
- can I use a 2X20v transformer without make the distortion grow, I would like to keep it at his optimum;
You can use a 2 x 20 V transformer, if your speakers are 8 Ohm and you use the TA package and you have an adequate heatsink.

With 2 x 20 V and 4 Ohm speakers you will run into thermal shutdown.
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Old 22nd November 2008, 01:37 PM   #32
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Thanks for replying.
My speaker is a diy two way made of a 8ohms tweeter and a 8 ohms mid-bass, so I guess its impedance is around 4/6ohms...
I don't need so much power, as long as these speakers have a sensivity around 92/93db...

The LM datasheet gives distorsion vs power for a rated voltage of 25v ; what is the transformer voltage required to rich 25 after after PSU ?

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Old 25th November 2008, 04:26 PM   #33
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There are several factors in a power supply that change with the current, with temperature, and with the components you use. That makes it difficult to reach a specified voltage exactly. The current is dynamic, temperature depends on current, operating time and ambient temperature, and each component has it own characteristics.

A 2 x 20 V transformer would lead to more or less ±25 V at nominal output. But you should consider that
  • - mains voltage can be up to 10 % above nominal, which leads to 10 % higher secondary voltage.
    - transformer regulation can lead to higher secondary voltage at low or no load.
    - PSU capacitors lead to higher voltage at low or no load.
If all those factors come together, the rail voltage can become too high for the LM1875. So you should prefer transformers with 2 x 18 V or less.
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Old 25th November 2008, 07:31 PM   #34
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Thanks

I think I'm going to use a 2X15v 120 or 160 va, 120 va if it's enough. I don't want to overdrive the LM1875 chip, and I guess this will allow me to use an average size heatsink.

I was also wondering about the heatsinf insulation : do I need a mica piece between the LM box and the heatsink ?
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Old 26th November 2008, 04:44 AM   #35
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Yes, on the insulator, and don't forget the head of the screw needs to be insulated from the tab on the device also. A nylon "hat" washer with the length trimmed to fit the thickness of the tab.
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Old 26th November 2008, 06:51 AM   #36
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Hi,
and thanks again for supporting, I'm trying to put up everything clear before I start to solder. So the next question is about the optional components on PSU and Amp board ; what is the purpose of those optional networks :

- the zobel network on amp board
- optional resistor to ground before C1 on amp board
- optional resistor between input ground and uotput ground
- snubber capa on PSU board

I should I choose for one or either ; all this is newbie wondering : I'm starting here my first diy amp project...
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Old 26th November 2008, 03:30 PM   #37
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I just left them all out, mounted the zobel components at the speaker terminals, where they are easier to change out, to evaluate their effect. It works fine. I wish Brian or someone would post the board schematic, just for reference. (leaving R1, R6, R5, and C7 open on mine) On one channel I even jumpered across the C2 pads instead of putting the cap in, to see if I could hear a bass difference. I'm not using test speakers with much bass response though, so I'll put it back in.
Cheers!!
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Old 26th November 2008, 04:12 PM   #38
vonfilm is offline vonfilm  United States
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Brian GT said:

Quote:
Note that you will wire your input to the top pad of C1. This avoids having to use a short for C1, and should be a cleaner wiring solution.
Please expand on this for a newbie. If I wire input to C1 does SG remain the same? If C1 is unpopulated will IN no longer work? If I want the possibility of adding C1 in the future should I jump C1 and wire input to IN?

So far I have built my amp with the minimal parts. I noticed Zymurgy's version seemed to have all the optional parts. Couls you explain the pros and cons of the optional parts?

Brian GT said:

Quote:
-For the Avel Lindberg transformer in the US, connect the Blue and Violet wires to the AC H connection on the switch, and the Gray and Brown wires to the AC N connection on the AC input
terminal.
My IEC switch is marked N and L instead. Should I assume L is the same as H? I am now going to use the Avel transformer recommended on the Chipamp.com website.

Any word on the long awaited 1875 manual? Thanks for all the help!
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Old 26th November 2008, 07:59 PM   #39
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I think I'll first give a try without optional stuffs ; there's no need to put jumpers instead of either of them, indeed ? (except for R6 on amp board).

I still don't know the transformer's size required : I wish I could downsize price, by using a 120 va toroïdal transformer instead of a 160va, but I don't want to loose dynamics, or something else.

This amp will drive a pair of 8" single driver (hemp acoustics FR8C) rated at 95 db, but maybe down to 92-93 db in the mid range when they will be crossed with a pair of 90 db hiquphon tweeters. A middle or high middle sensivity I guess. So, am I wrong with 120va ?
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Old 28th November 2008, 06:48 PM   #40
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Quote:
Originally posted by androuski
what is the purpose of those optional networks :

- the zobel network on amp board
- optional resistor to ground before C1 on amp board
- optional resistor between input ground and uotput ground
- snubber capa on PSU board
  • - The Zobel is there to create a low impedance or high load at high frequencies. It helps against oscillations that could damage the amplifier.
    - The 1 M resistor is a bleeder resistor. It is there to deal with any leakage current that may come from C1, and makes sure that no DC potential builds up, when nothing is connected to the input.
    - The resistor between input and output ground can help in some cases against hum issues.
    - Their name explains the function of the snubber caps. They snub noise, e. g. the high frequent ringing that is caused by the diode's switching in the rectifier.

Quote:
Originally posted by androuski
I still don't know the transformer's size required : I wish I could downsize price, by using a 120 va toroïdal transformer instead of a 160va, but I don't want to loose dynamics, or something else.

This amp will drive a pair of 8" single driver (hemp acoustics FR8C) rated at 95 db, but maybe down to 92-93 db in the mid range when they will be crossed with a pair of 90 db hiquphon tweeters. A middle or high middle sensivity I guess. So, am I wrong with 120va ?
You can expect around 15 W of output power per channel with a 2x15 V transformer and those 8 Ohm speakers (decent cross-over provided). Reasonable transformer sizes are 1-3 times output power, so with 120 VA you are already on the very generous side. Anything between 30 and 90 VA would do for a stereo set-up. A bigger transformer may improve the sound, if you listen very loudly, because it remains out of saturation even on peaks. And it has advantages, when it comes to regulation.
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