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Bijou All Tube Futterman Headphone Amplifier - Page 121

post #1801 of 3976
11/5/08 at 4:07pm
Yes, everything is pointing me to shunt pot... thanks, all, just gotta figure out which way to go. Sadly, I did not choose the nfb path, so that's out.

And, let's not forget I couldn't wire up my attenuators in the first place... but, I do like the approach of shunt in the abbreviated sense, but think the Goldpoint solution



more elegant, assuming it will work the same with my attenuators. Now we will see what resistors I have close to 90k / 10k - will something boutique help here?
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post #1802 of 3976
11/5/08 at 8:19pm
I would reconsider the NFB You can use a fixed path, it should be easy to add.

I'm a big fan of using enough gain for your needs. A hot source with a balanced amp can be way too much for any person. I'd guess your source is probably putting out 4-6Vrms? That's a big voltage swing.

I'm not really a fan of attenuating the source signal to an extreme degree, followed by huge gain in an amp. Less gain, less attenuation, yields finer control with your potentiometer.

To answer your question, perhaps. Do you believe in boutique resistors? If so, then yes.
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post #1803 of 3976
11/5/08 at 9:34pm
I think you hit the nail on the head Holland. Pabbi will need to put a fixed NFB resistor in place of an NFB pot, along with the others necessary, and I would be careful to add just enough. Too much NFB is also a very poor sounding solution. Then with the reduced gain, the attenuator can use smaller values, assuming you go with the goldpoints. The intermittent On/Off of the power suggests a short, and I never had that problem. Check the tape that wraps around the heater wiring and make sure it's not allowing the heaters to touch ground there. Edit: the NFB could be tried last, because then all of those boards have to be pulled. You could try the goldpoints with larger resistance first to see if the sound is worthy.
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post #1804 of 3976
11/5/08 at 9:36pm
Quote:
Originally Posted by pabbi1 View Post
Now we will see what resistors I have close to 90k / 10k - will something boutique help here?
you must support the locals.. they do sell direct, and best resistors out there.

Texas Components - Precision Resistor & Microelectronics
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post #1805 of 3976
11/5/08 at 10:42pm
I had the cage off my amp today, checking the PS voltage. After about an
hour, I noticed that the 270 DAX was running very warm; enough that I
couldn't hold my hand on it for more than a couple seconds. Is this normal, or
do I need to look at changing to the 370?
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post #1806 of 3976
11/5/08 at 10:44pm
Quote:
Originally Posted by amphead View Post
I think you hit the nail on the head Holland. Pabbi will need to put a fixed NFB resistor in place of an NFB pot, along with the others necessary, and I would be careful to add just enough. Too much NFB is also a very poor sounding solution. Then with the reduced gain, the attenuator can use smaller values, assuming you go with the goldpoints. The intermittent On/Off of the power suggests a short, and I never had that problem. Check the tape that wraps around the heater wiring and make sure it's not allowing the heaters to touch ground there. Edit: the NFB could be tried last, because then all of those boards have to be pulled. You could try the goldpoints with larger resistance first to see if the sound is worthy.
Yeah, the thought of pulling all the boards was not my idea of a good time, especially if I didn't get the value right the first time - but, adding sockets for resistors has appeal. I was actually just showing the Goldpoint solution for how I was considering rewiring the existing attenuators.

So, what value would the suggestions be for NFB? Does this mean _changing_ R1 (YUCK!!) or just adding R12 or... ?

I'm letting this sit for a day or two - I have some finish work to do on a BH mod...
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post #1807 of 3976
11/5/08 at 10:49pm
Funch, yes, that is normal. They seem to run very warm. If it is frying egg warm, that would be too much of course. You could try a separate tranny for heaters, to reduce the temp on the PS transformer.

Pabbi, it will need all of the resistors as shown on Alex's website. The reverse of this:
* Eliminate the 250kΩ feedback pot(add 125k resistor)
* Remove (or don't install) R12(install R12)
* Change R1 from 10kΩ to 300Ω(change R1 to 10K)
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post #1808 of 3976
11/5/08 at 10:53pm
OK, thanks. I'll just leave it alone, unless it goes boom.
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post #1809 of 3976
11/6/08 at 9:12pm
Chobint, here's the schematic you were looking for.......



Pabbi, let me know if there is anything I can do to help. I hope that damn intermittent ON/OFF is found. The amp was working so well, I just don't get it.
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post #1810 of 3976
11/6/08 at 11:00pm
Thanks for the schematic Amphead. As usual my initial guess was wrong, so you must forgive me for asking another question. It appears that the module will be powered by two 9v batteries in series(?). Does that mean, alternatively, that I could use an 18v wallwart? As I mentioned earlier in PM's, the negative voltage thing throws me for a loop.
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post #1811 of 3976
11/7/08 at 12:27am
Chobint, if you want to use wallwarts, then it will be two 9 volt supplies. One of the supplies has the positive tied to ground and the other has the negative to ground. The one with positive tied to ground will be connected with the negative tied to pin 11. The one wired "normally" with negative tied to ground will be connected with the positive tied to pin 4. Edit: you can use an 18v wallwart with a voltage divider if you want.
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post #1812 of 3976
11/7/08 at 12:35am
makes sense. i will try that. Thanks again
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post #1813 of 3976
11/8/08 at 2:31am
runeright,
i need help finding an appropriate heatsink for the Q2 mosfet, assuming i decide to leave the multiplier add-on on. i'm not sure that my original idea of stacking another 1.5" Standard Products Found on top of the existing one will work - yes or no? space is tight on the PS board and finding a heatsink with an appropriate thermal "resistance to ambient" is difficult with the space limitations.

i was wondering if this one would work: Standard Board Level Heat Sinks

the Tj i calculated is a bit on the high side though, at 89 C :
Tj= Ta + (P(Rjc+Rcs+Rsa))
where,
Ta = 50 C (an assumption based on the fact i was getting 36 C with the chassis cover on, but only one PS board and one EZ81.)
P = 4 W
Rjc = 1.79 (Fairchild data sheets - http://www.fairchildsemi.com/ds/FQ/FQP4N50.pdf)
Rcs = 0 (mosfet not isolated with pads, only thermal paste)
Rsa = 8

do my maths look right?

soldering C3 on the underside of the PS board could free up some space for larger fins as well.

i'm also considering DIY some custom aluminum heatsinks or trying to mount the Q2 on the underside, mounted directly to the bottom of the chassis.

thanks!
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post #1814 of 3976
11/8/08 at 5:04am
ooh, ooh... this may work: Wakefield 634 series

Digi-Key - 345-1027-ND (Wakefield - 634-20ABP)

and scroll down to pg. 15 on the data sheets:
http://www.wakefield.com/PDF/board_level_heat_sink.pdf
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post #1815 of 3976
11/8/08 at 7:45am
Spent some time modding the bijou this morning.

Socketed r2 for tube rolling.

changed output bypass to obbligato films.

rolled in some 6n1p EB.

I'll let it burn in for some hours and report back.

maybe its just new tubes that need to break in.... but bass is huge compared to the 6922's.
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