Either pot should work, I don't use big ones because I personally don't really find it necessary to spend more than $5 for a good Alps RK09. I generally use mine at max volume anyway and use the source to set volume. You will find the RK09 quite sufficient. Some people will spend $100 on a stepped volume attenuator with 0.1% 10ppm Susumu resistors etc... 
267 ohm resistors will work ok for R124 & R125, right? It's the only value I can find the Dale CMFs in at the correct size & wattage. I'm using Mouser BTW, but I couldn't find anything even in the right neighborhood at Digikey.
Also I can't find any 1W (or greater) CMFs between 260-280 ohms for R115-119 & R121-123. I found some 2W 270ohm in the Vishay CPF (flameproof) series. Does anyone have experience using that series of resistors in audio circuits? The specs line up as good or better than the CMF series; I just want to make sure they don't sound like crap for some reason.
Also I can't find any 1W (or greater) CMFs between 260-280 ohms for R115-119 & R121-123. I found some 2W 270ohm in the Vishay CPF (flameproof) series. Does anyone have experience using that series of resistors in audio circuits? The specs line up as good or better than the CMF series; I just want to make sure they don't sound like crap for some reason.
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The four 1W res on source of MOSFETs are important to use good quality 1W metal thin film. Don’t use metal oxide thick film, or carbon composition, or wire wound here. I would avoid “flame proof” typically is a power resistor and might be inductive or high distortion. Try to get close to 270ohms does not have to be exact.
All the resistors in the audio path to the JFET and above and below the JFET should be metal thin film as well. Even cheap no name (sky blue color) metal thin film works here.
The gate snubber on the MOSFET could be Takman Rex carbon film for smooth sound.
R141-134 doesn’t matter what kind you use.
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There are CMF60s available in 221R and 330R.
So 2 at 330R plus 2 at 221R gives Req of 66.18R. Which is 1.95% lower than nominal 67.5R. Is that close enough? But then am I going to be dissipating more than the rated 1W through the 221R resistors? Hmm I need the voltage across those resistors to calculate that. Do you have a rough idea xrk? (or exact, I'm sure you have this in SPICE)
Or is this method a bad idea?
So 2 at 330R plus 2 at 221R gives Req of 66.18R. Which is 1.95% lower than nominal 67.5R. Is that close enough? But then am I going to be dissipating more than the rated 1W through the 221R resistors? Hmm I need the voltage across those resistors to calculate that. Do you have a rough idea xrk? (or exact, I'm sure you have this in SPICE)
Or is this method a bad idea?
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Just get the Dale CMF60 in 280ohms (4x per channel) and you will be fine. 1200 in stock at $0.47ea.
CMF60280R00BHEB Vishay / Dale | Mouser
CMF60280R00BHEB Vishay / Dale | Mouser
A few more questions...
1) It looks like BF862 is an obsolete part. Is there a good replacement, and if so, what is it and does it require and resistor values to be changed from what's on the BOM?
2) for V111 and V112, the BOM shows IRF610 in Column 2 and IRF512 in Column 3. Does that mean either will work, with the resistor values on the BOM? It looks like IRF610 is currently available, while IRF512 is obsolete.
3) Also, I saw you mention that the next run of DCA boards might have solder mask over all the vias. If I wanted to wait for that next version of the board with the "tented vias", how long might I be waiting?
1) It looks like BF862 is an obsolete part. Is there a good replacement, and if so, what is it and does it require and resistor values to be changed from what's on the BOM?
2) for V111 and V112, the BOM shows IRF610 in Column 2 and IRF512 in Column 3. Does that mean either will work, with the resistor values on the BOM? It looks like IRF610 is currently available, while IRF512 is obsolete.
3) Also, I saw you mention that the next run of DCA boards might have solder mask over all the vias. If I wanted to wait for that next version of the board with the "tented vias", how long might I be waiting?
Over in the Yarra thread we discuss the new setpoints for PCA/DCA daughterboard (same as DCA in principle) either the 2SK209GR or 2SK170BL(available from GB’s) or LSK170BL. Those 2SK209GR’s are Toshiba JFETs and still available.
The YARRA Preamplifier/HPA for Melbourne DB Group Buy
Use IRF610 or Fairchild FQP4N20 work very well too.
You will wait a long time since I have a lot of the original boards left.
The YARRA Preamplifier/HPA for Melbourne DB Group Buy
Use IRF610 or Fairchild FQP4N20 work very well too.
You will wait a long time since I have a lot of the original boards left.
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I've got a question about Test Equipment -
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When you're building & troubleshooting an Amp, what sort of Oscilloscope do you use?
.
How many channels, and how much bandwidth is actually needed for audio circuits?
.
I'm considering one of the 2-Channel, 25-MHz PicoScopes, that has an Arbitrary Waveform Generator. The O'scope software is available for Windows, Macs, and Linux.
.
When you're building & troubleshooting an Amp, what sort of Oscilloscope do you use?
.
How many channels, and how much bandwidth is actually needed for audio circuits?
.
I'm considering one of the 2-Channel, 25-MHz PicoScopes, that has an Arbitrary Waveform Generator. The O'scope software is available for Windows, Macs, and Linux.
For testing audio equipment, almost anything can work as the signals are relatively low bandwidth. When looking for oscillations, it is helpful to be able to see up to 20MHz or so. I like convenience of a stand-alone scope that is not tied to my PC via USB. It’s quieter and reduces possibility that the scope of connected to some hot mess high voltage can also fry the PC.
For about $200+ you can get a stand-alone 2 ch digital flat screen color display scope with 50MHz to 100Mhz bandwidth.
Popular brands are Rigol, Hantek, Owon, (other Chinese/Korean/Asian brands).
I have an Owon 100MHz and I like how it has controls that feel and behave just like a Tektronix that I used to use as work. It has more features than I need but is useful for some higher speed digital applications.
I have something similar to this one:
https://www.amazon.com/dp/B00CUN7RNI/ref=cm_sw_r_cp_api_fabc_AHyPFbS9TQAKB
For about $200+ you can get a stand-alone 2 ch digital flat screen color display scope with 50MHz to 100Mhz bandwidth.
Popular brands are Rigol, Hantek, Owon, (other Chinese/Korean/Asian brands).
I have an Owon 100MHz and I like how it has controls that feel and behave just like a Tektronix that I used to use as work. It has more features than I need but is useful for some higher speed digital applications.
I have something similar to this one:
https://www.amazon.com/dp/B00CUN7RNI/ref=cm_sw_r_cp_api_fabc_AHyPFbS9TQAKB
Post #276 is with regards to using the PCA as a daughterboard for the M2X. Since the M2X is about 15dB of overall gain, I looking for about 5-6dB of gain for the PCA (total of 20dB gain). As such I am sure there will need to be some fiddling of the resistors in series with the Drain and Source of the JFET, in addition to carefully looking at the harmonic profile.
Here is the schematic I am referring to:
https://www.diyaudio.com/forums/gro...fier-hpa-melbourne-db-buy-62.html#post5824862
Best,
Anand.
Here is the schematic I am referring to:
https://www.diyaudio.com/forums/gro...fier-hpa-melbourne-db-buy-62.html#post5824862
Best,
Anand.
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This will take an LTspice sim to adjust. It is not so simple. Same thing with the Hakuin. Because changing the gain requires adjustment of the resistors above and below the JFET and that changes the bias current and the harmoinic profile. I think it might be easier to throw away some gain with a simple output voltage divider and preserve your harmonic profile. Drop 6dB gain by putting two 470k resistors in series to ground at the output. Tie your M2X input from the mid point. Easy peasy.
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