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Old 12th November 2013, 01:09 AM   #21
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Ian, Bigun, I don't pretend to understand the detail of what you are discussing, but it seems to me that in order to preserve the P3A character, it is necessary to:
- Retain the CFP VAS
- Stick to one pair of outputs

Isn't it enough to evaluate the effect of changing design element (i.e. LTP to singleton input)? It would be great to focus some efforts on the TGM8 BOM, so that it reflected the recommendations from Sarkis and others in the P3A thread, with the board layout component spacings to suit.
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Old 12th November 2013, 01:48 AM   #22
AKSA is offline AKSA  Australia
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You know, if you rearrange the input and output stage, it really is a different beast and will sound very difficult.

What about using a quasi output, and using monster fets? I have have very good experiences with quasi output stages; they increase H2 and H3 and sound very warm yet do not lose resolution. And if you use mosfets, you can use IRFP450, which is a 500V, 12A 200W mosfet.

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Old 12th November 2013, 03:18 AM   #23
Bigun is offline Bigun  Canada
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Well I have to agree with all the good advice about not detracting to much from the original design whose sound signature we still wish to capture. So we should leave the VAS alone. Which means it stays as the original, a simple single device.

As for the output stage, if I leave the MOSFET option in the design it can be built with or without them. Assembled without them and it will be just as the original. Having this option still appeals to me for now - I'll see if the pcb layout makes me change my mind on keeping it in.

BOM questions already - well then, which resistors do we want to maintain as through-hole to allow for special parts-tweaking, whilst the rest can be 1206 parts for a compact layout ?

I've uploaded the spice file for those who like to play - I will need to review the parts values a bit, especially the capacitors so that I size them correctly.
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Last edited by Bigun; 12th November 2013 at 03:26 AM.
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Old 12th November 2013, 03:43 AM   #24
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Hello Hugh

If you opt for a MOSFET output stage using the IRFP450, then why don't use its complimentary pair (IRFP460) instead of a quasi complimentary design?

EDIT: apologies, I seem to be mistaken: the IRFP450 has no compliment.

Last edited by Ranchu32; 12th November 2013 at 03:49 AM.
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Old 12th November 2013, 03:57 AM   #25
AKSA is offline AKSA  Australia
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Ah, because the H2/H3 levels are higher and it sounds tubey........

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Old 12th November 2013, 04:39 AM   #26
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Bigun, why not use all 1206 resistors? I find it easier to desolder SMT resistors than TH, so parts tweaking shouldn't be a reason to use TH in those locations...

Can you please put the filter caps onboard and the DC protection trigger too (+ relay if there is room).
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Old 12th November 2013, 07:47 AM   #27
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Quote:
Originally Posted by Bigun View Post
BOM questions already - well then, which resistors do we want to maintain as through-hole to allow for special parts-tweaking, whilst the rest can be 1206 parts for a compact layout ?
R27/R28 (VAS bootstrap) and R33 (Zobel) should have 10mm footprint so that 1W MOX or 0,5W Metal film TH resistors could fit. That's where we anticipate possible higher dissipation. Also, it would be nice to use 10mm footprints for signal input shunt and series resistors so that low noise metal film can be used in these positions too. I do not know if 1206 SMD is available as low noise metal film. If it is, than input resistors also could be SMD. 0R33 "emitter" resistors should have big footprints too, butr they can be vertically mounted to save pcb space.

Last edited by ivanlukic; 12th November 2013 at 07:49 AM.
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Old 12th November 2013, 01:44 PM   #28
Bigun is offline Bigun  Canada
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The 0R33 resistors can also be SMT if we want, Hugh uses these on some of his designs (although not all of them) and they are fairly beefy. I've used the Dale WSR series 4527 sized parts, good for 2W and 1% tolerance.

The 1206 resistors I have used before are all (thick) metal film (Stackpole from Digikey), but some folk like to use other stuff. I'll start out with 1206. I have used larger sizes when higher power dissipation is needed so that should be fine with SMT also. For the zobel I do recommend a through-hole part just because.

Filter caps - OK.

dc-protection - I don't like relays because there are very few of them available that you can trust to open a high current to an inductive load. Some people say the contacts get dirty and affect small signal performance too. My preference is to make a solid-state relay from a pair of NMOSFETs. With very low Rds on they will have no influence on the signal, they turn on and off very fast and can switch all the load the current we need. They don't need to dissipate any power so we can use a pair of TO-220 or SMT packaged devices without any heatsinking.

Do we want base-stoppers on the output devices? or on the drivers ? Rod doesn't use them but I've always included them in the past with EF output stages but I've not used a CFP output before. ?
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Last edited by Bigun; 12th November 2013 at 02:14 PM.
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Old 12th November 2013, 02:05 PM   #29
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Originally Posted by Bigun View Post

dc-protection - I don't like relays because there are very few of them available that you can trust to open a high current to an inductive load. Some people say the contacts get dirty and affect small signal performance too.
Absolutely right! They degrade sound after only a few years and usually it is impossible to open them for cleaning. Only problems with them.

I think that we probably do not need base stopers.

Last edited by ivanlukic; 12th November 2013 at 02:13 PM.
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Old 12th November 2013, 11:31 PM   #30
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Bigun, I like the idea of using a solid state relay - I've just never used them personally. Is this the sort of thing you are suggesting:

PSMN013-100BS,118 NXP Semiconductors | Mouser

How about those filter caps any ideas? :-)
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