Hi Fran,
I obtained mine from eBay (bad idea, one was defective), ampslab and zhouFang
http://www.diyaudio.com/forums/vend...u-always-wanted-but-hard-get-they-here-9.html
I used the usual values and it works fine. You can see EUVL's posts or TeaBag's blog for alterations. In consideration of EUVL's recommendations I did use jFets with Idss of 6.9mA for 2sk170 and 6.5mA for 2sj74.
Hi Tea-Bag,
Vishay bulk metal foil trimpots (same family as the "nude") can be had at:
Michael Percy's (page 12)
Texas components
or this guy on eBay where I've bought several:
2 Vishay 1202Y 2k 25 Turn Mil Spec Trim Potentiometers - eBay (item 200454699643 end time Jun-24-10 07:56:45 PDT)
Look for 1280G (smaller, 3/4W) or 1202y (larger, 0.5W). I've used both, but the 1202Y has longer leads so may be easier to use despite being larger.
If you can find the time, it would be nice if you could give your impression on the irfp240/9240 parts.
Thanks for the tip.
The 'nude' tx2575's to my ear's are a great resistor to have in the signal path. I don't regret there expense.
Hi thanh1973,
Thanks for the interest in my impressions. I do have the IRF parts somewhere around here. However, after the FQA's, Toshiba's and Hitachi/renesis 2sj162/2sk1058, my motivation is a bit lacking at this time. I know the IRF parts have their fans such as UMUT and Peter Daniel, but from Nelson's description I doubt I would prefer them in my system. Other projects beckon, such as a Buffalo II kit on my desk 🙂 I will probably try them but it may be a while.
Just had an idea.
For those people that feel they need more voltage gain, what about putting a 50 Ohm linear pot/channel (probably needs to be the wire wound variety) in series with the 50 ohm (ie 2 x 100 ohm in parallel) feed back resistors.
This might actually be a bad idea - I have no idea how much inductance would be added and what effect it might have on the stability of the amp.
But for people who like to experiment it is probably worth a try. I think it would be interesting at least.
The other mod I highly encourage is to replace the 10 ohm jfet source resistors with a 20Ohm multi turn potentiometer start at the middle setting and play by turning it 1/2 a revolution clockwise then set it back to middle position and then turn it 1/2 a revolution and anticlockwise - listen to the difference of the sound while adjusting.
Instead of removing the resistors you could put a 1k pot in parrallel, although it will take a few turns each direction rather than 1/2 a turn.
For those people that feel they need more voltage gain, what about putting a 50 Ohm linear pot/channel (probably needs to be the wire wound variety) in series with the 50 ohm (ie 2 x 100 ohm in parallel) feed back resistors.
This might actually be a bad idea - I have no idea how much inductance would be added and what effect it might have on the stability of the amp.
But for people who like to experiment it is probably worth a try. I think it would be interesting at least.
The other mod I highly encourage is to replace the 10 ohm jfet source resistors with a 20Ohm multi turn potentiometer start at the middle setting and play by turning it 1/2 a revolution clockwise then set it back to middle position and then turn it 1/2 a revolution and anticlockwise - listen to the difference of the sound while adjusting.
Instead of removing the resistors you could put a 1k pot in parrallel, although it will take a few turns each direction rather than 1/2 a turn.
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If replacing the jFet source resistors (R1&R2) with a potentiometer, make sure it is at least 1W+
The current in this region will not exceed idss of the jfet. That will mean, probably between 7mA to 12mA for most people, so I think you could get away with a 0.5W trim pot.
1- There is a ratio to respect: R5//R7 / R1
2- You are forgetting the feedback current through R1 And R2
2- You are forgetting the feedback current through R1 And R2
Yes you are right, thanks.
I was trying to remember the circuit from memory (I have a memory like a sieve).
I was trying to remember the circuit from memory (I have a memory like a sieve).
Let's use the 12mA of your example.
Power = 0.012^2 * 1000 = 0.144W.
A 500mW pot would be at <29% of maximum power rating.
the two 100r in parallel are required because of the full output signal applied to these very low value feedback resistors.
Do not put any pot/rheostat in series with these resistors.eg.
Lets add a 50r resistance to the two parallel 100r and assume the maximum output signal is 22Vpk. The power dissipated in each of the resistors is calculated as follows.
Total Voltage 22Vpk
Total resistance =100r.
Total current = 220mApk.
P100r = .11^2 * 100 = 1.2Wpk
Prheostat = .22^2 *50 = 2.4Wpk
The 50r rheostat dissipates twice what each 100r dissipates.
Would that lead you to using a 10W rheostat compared to a 5W 100r?
Bob corrected me on that one back at post 671.
I really should have thought it through properly, and had a look at the schematic (I haven't looked at it for over 9 months now).
I hope I haven't confused anyone here.
I really should have thought it through properly, and had a look at the schematic (I haven't looked at it for over 9 months now).
I hope I haven't confused anyone here.
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Probably the best solution would be to switch in a series power resistor, (maybe a triple throw switch with 0 Ohms, 25 Ohms, and 50 Ohms in series with the feedback resistors).
Hard to say. Quite a few people prefer the sound of the
IRFP240/IRFP9240 combination over the lower distortion
Fairchild parts. I haven't sent any Toshiba's into the field,
but I would expect a scattering of preferences between these
three parts.
😎
The truth is that i like IRFPs but i listened to IRFPs long time ago.But my friend(I trust his ears) liked IRFP's because of its slower response and more harmonics(he likes tube amp sounds and harmonics)
I prefer fairchilds to toshibas(It is because toshibas sound faster and gives deeper bass) but these so much details make headache to me.Maybe i am wrong, I will make another comparisions.
I prefer fairchilds to toshibas(It is because toshibas sound faster and gives deeper bass) but these so much details make headache to me.Maybe i am wrong, I will make another comparisions.