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Old 12th April 2009, 01:03 PM   #341
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by njepitt
OK, here's a drawing of the circuit I'm using. I didn't put on the values of the bias resistors, since I'm using trimpots for these (at least for the time being). These are adjusted to give 550mA bias, as the Monstre articles recommend. (Although I suppose the recommended value depends on the output transistors too, right?).

I don't use PCBs (at least not yet), all done p2p.

It's not too hard to reduce the resistors on either side of the trimpot (which would reduce it form 460R to, say, 300R). What do you think?

Cheers
Nigel

regarding "loudness" ( gain ) issue - double check values of 220 and 10 ohms resistors in feedback net ,same as functionality of feedback net ( is it connected at all , where it need to be ) .

excessive gain and noise are signs that you're running that amp open loop ........ and fact that you have same culprit in both channels is sign that you made some system mistake ... (not connected track or similar)
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Old 12th April 2009, 01:12 PM   #342
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actually, to support what Zen mod says,

I remember there is some trick with the location of the 10 Ohm resistor
its location is shown incorrectly on the pcb overlay pic layout in the class a amplifier site and should be connected as per the circuit diagram.

(that is from the pot wiper to earth)

-Dan
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Old 12th April 2009, 02:01 PM   #343
njepitt is offline njepitt  United Kingdom
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Thx for the replies. I'm not sure what "open loop" means... Am I right in guessing it simply means the feedback loop isn't working? (which is the second half of ZenMod's reply) Anyone have a reference where I can read up? Certainly I can (and will) check the remarks about the 10R and 220R resistors, and the feedback circuit as a whole.

Returning the trimpot to 100R sounds a backwards step - after all things sounded dreadful before raising it. The original circuit (which uses Y grade jfets) uses 100R, other circuits I have seen using GR grade use 200R. I simply guess-timated the value. Reducing it to 200-300R sounds a more plausible thing to try.

Nobody seems to think the output transistors are the culprits, right?

I'll report back.

Cheers

Nigel
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Old 12th April 2009, 02:06 PM   #344
Zen Mod is offline Zen Mod  Serbia
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@njepit :

simply check have you proper conection of 220R and 10 R , between output and trimpot ( between sources of jfets )

open loop - means - disconnected feedback net ( 220r-10R - pot ) , when amp have entire possible gain ..... , not decreased with feedback .....

in that case - you'll have excessive gain and noise

if you have feedback connected as needed , any swap of output transistors will have just diminutive influence , not massive one .....
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Old 12th April 2009, 02:13 PM   #345
njepitt is offline njepitt  United Kingdom
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Quote:
Originally posted by Zen Mod
@njepit :

simply check have you proper conection of 220R and 10 R , between output and trimpot ( between sources of jfets )

open loop - means - disconnected feedback net ( 220r-10R - pot ) , when amp have entire possible gain ..... , not decreased with feedback .....

in that case - you'll have excessive gain and noise

if you have feedback connected as needed , any swap of output transistors will have just diminutive influence , not massive one .....
Thanks ZenMod. Very clear explanation. I'm checking the 220r -20r network as I write this....

Two questions - the trimpot is evidently part of the feedback circuit. Presumably its value therefore affects the feedback loop. So if it's value is to high there is too little feedback. Would this have the same kind of effect (although to lesser degree) as no feedback? That is, force the amp into higher gain than is desired? The second question if what happens if the two bias resistors are unbalanced? This may be an issue since I am using trimpots.

Cheers

Nigel
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Old 12th April 2009, 05:14 PM   #346
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Default hiraga class a amp

Hi! I'm new here, but 4 yrs ago i built this amp, but finding the parts is like needle in the haystack, but its worth it, very open, very natural. big factor is the power supply, instead i use capacitance multiplier i got from Rod Elliots site. HUm was eliminated. But what really gave me goosebumps is when i built the 1969 version of JLH modified to dual rails by Rudy van Stratum. I still use this amp to this day, my HIRAGA was now in the hand of a friend as a gift. When he tried the HIRAGA his commercial amp is now in his shelves, collecting dust.
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Old 12th April 2009, 07:58 PM   #347
DIGORA is offline DIGORA  Belgium
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Quote:
Originally posted by njepitt
Hi Dan,

Yes, I realised the 2N5462 wasn't ideal - I used it in the first version simply because it was what I could get my hands on. Now that I have 2SK170 and 2SJ74 (BL grade) I am using these in the second (now third) version.

Since I posted I have also isolated another possible problem. The trimpot I am using is 100R, which according to some posts above is correct for the Y grade jfets, but too low for the BL grade (which have higher Idss...). So I am going to pop in two fixed resistors on either side of the trimpot to see what difference this makes. (I am guessing at 100 to 180R on each side to give a total of 300 to 460 R on the trimpot).

I am a little confused by your remark about Cob of the Toshibas. The datasheet for the 2SB754 (which is the original device) gives typ. 300pF, whereas the 2SC5200 shows typ. 200pF... While out component shopping I picked up two pairs of MJW3281/1302 to try in place of the 2SC5200/1943, but I see from the datasheet that their Cob is higher still (max 600pF is given). Worth trying? (You are certainly correct that the power rating of the 2SC5200/2SA1943 is far higher than necesssary here. I used them because I had them....)

I'll send you a PM about your original transistors.

Cheers

Nigel

The MJE15030 could be used ?
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Old 14th April 2009, 05:38 AM   #348
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Hi all,

Just a few pics of original le monstre some "eye candy"
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File Type: jpg le monstre - interior.jpg (19.2 KB, 508 views)
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Old 14th April 2009, 05:41 AM   #349
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more pics
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File Type: jpg le monstre - supply.jpg (36.7 KB, 488 views)
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Old 14th April 2009, 05:56 AM   #350
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more pics
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File Type: jpg le monstre -charging setup.jpg (25.2 KB, 491 views)
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