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I was surprised when I learned that H2/H3 behavior was highly dependent on volume level. At lower volumes, it is difficult to measure/discern any differences at all. But things "bloom" a bit more (and will vary) as you approach the -10dB to 0dB range. While I won't be pushing this level into my Aleph-J amp very often, it does happen with my MoFo amp because it needs some voltage gain in order to function.
My first goal was to adjust P3 to minimum distortion and test how that sounds. It seems that at the -0.8 dB level for my measurements that H2 and H3 is about equal. These kind of adjustments / measurements are time consuming.
Do you re-check bias and offset after P3 adjustment?
If P3 changes bias/offset and you then re-adjust these then your P3 measurements are probably "gone"?
I use BA3 pre for MoFo in the moment and my thoughts was that too much H2 is not a good thing…...assuming that MoFo is mostly H2.
Do you re-check bias and offset after P3 adjustment?
If P3 changes bias/offset and you then re-adjust these then your P3 measurements are probably "gone"?
I use BA3 pre for MoFo in the moment and my thoughts was that too much H2 is not a good thing…...assuming that MoFo is mostly H2.
Yes, you need to readjust offset after adjusting P3. It will effect offset and bias.
Think of it this way...P3 effects the symmetry of the jfets drain values, adjusting P3 balances or unbalances how they are working together. P1 and P2 effect the incoming voltage to the jfets.
It's best to check harmonics after DC offset is nulled but I don't believe the harmonics will change much when zeroing offset with p1/2 assuming it's not extreme.
50K is probably not ideal but will work. Seems like 10K or 25K would be more appropriate if your source can handle it.
This is what's used in Pass XP preamps:
VCU, Hi-End MUSES(R) Micro Volume Control Board (stand alone) | academyaudio
Of course, you will need to power it w/12V.
This needs to be a simple pot or other simple way to get a balance control without adding a bunch of hardware.
I think you will find after some research the muses may actually be the simplest solution. It is a stand alone board, no LCD panel or anything needed. It just needs to be powered with 12V (use your existing supply and a 3 legged reg). It's the size of the existing pot and will almost certainly sound better than a two-pot solution. And, compared to two good pots, it's cheap.
But here are some passive solutions:
ESP - A Better Volume Control
I did not realize that there is a micro version of the Muses. Thanks for the pointer.
Ok. I have now 3 DMMs so it will be easier to adjust. Will be interesting to see how much offset and bias has drifted by adjusting P3. The P3 needed some adjustment to get to lowest distortion. Laster I will try other settings. I was not "blown away" with the sound after P3 adjust so maybe I will find the reason…….I am using my 300B with passive pre in the moment to get a kind of "sound reference".
It could be an endless "adjustment loop"......if I after adjusting bias and offset again need to check distortion and readjust P3.....and so on......
Hello,
Nice and compact solution the VCU, Hi-End MUSES® Micro Volume Control Board.
I was contemplating this solution since is bundled with input selector too.
relay volume controller
Anyone has used a similar solution? Is compact enough ; but does it have any downsides?
Nice and compact solution the VCU, Hi-End MUSES® Micro Volume Control Board.
I was contemplating this solution since is bundled with input selector too.
relay volume controller
Anyone has used a similar solution? Is compact enough ; but does it have any downsides?
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Hi escucalin,
I have this one. Works perfectly.
Bought it as a complete "passive pre" mounted in a chassis similar to the one linked on that page you referenced, and complete with transformer for powering the board and the relays.
I removed the transformer and the IEC inlet and built an Aikido tube pre into the chassis along with the volume control and channel selector. External PSU connected with an umbilical. I am very happy with the pre-amp.
One thing to consider: Input impedance of the volume control is a constant 50 kOhm, but the output impedance varies with volume setting. It is a few kOhms at lower settings (higher signal attenuation), but gets to a few tens of kOhms at settings near the top of the range. (I don't remember exactly, but I think it goes up to something like 25k).
So it's best to follow it with a buffer or gain stage with high input impedance.
Best regards,
Claas
I have this one. Works perfectly.
Bought it as a complete "passive pre" mounted in a chassis similar to the one linked on that page you referenced, and complete with transformer for powering the board and the relays.
I removed the transformer and the IEC inlet and built an Aikido tube pre into the chassis along with the volume control and channel selector. External PSU connected with an umbilical. I am very happy with the pre-amp.
One thing to consider: Input impedance of the volume control is a constant 50 kOhm, but the output impedance varies with volume setting. It is a few kOhms at lower settings (higher signal attenuation), but gets to a few tens of kOhms at settings near the top of the range. (I don't remember exactly, but I think it goes up to something like 25k).
So it's best to follow it with a buffer or gain stage with high input impedance.
Best regards,
Claas
I re-checked and re-adjusted offset and bias. The offset was drifted. I had 2.5V and left and 1.5V on right channel. So now I am back again to a few mV on both channels. I wonder how that could influence the sound. Will next time I power it on together with MoFo. I am on 300B SE in the moment....
Now I switched back to MoFo + BA3-pre after re-adjust of offset and bias. It was a positive experience as sound was not very different from 300B SE. MoFo has a "lighter" top end and a bit more punch in the deep bas. I have some pipe organ music I use for the comparation. 300B may have more "texture" in the bas sound....."crispyness"....difficult to explain but something I notice every time I switch to 300B.
I can also recommend Clarity Caps and yes they are better as you go up the price scale...
Best bargain are Russian PIO, that is what I used in my BA3 as Preamp
Why would anyone use paper in oil for anything else than a distortion generator?
paper in oil especially Russian ones seem to contradict the art or thought that is invested in these simple and gorgeous Pass circuits.
paper has a limited lifespan and once is gone acidic ; the capacitor goes DC leaky and among other things wildly nonlinear across audio band or any band by that matter since they become unable to do the only task they were designed(blocking DC).
See Mr C. easily verifiable video for a basic image (which is one i have as well): YouTube
So if i'm mistaking i would really like to learn how something like russian pio could even make it in an high fidelity audio equipment and why.
One could argue that the BA-3 as a preamp is just a distortion generator. There exists many, many other designs with far less distortion.
It all comes down to the sound you prefer. Most listeners and designers on this forum are not chasing minimum distortion but rather subjective sound quality. In general, PIO caps sound "round", or maybe what some describe as "tubey". It depends on the cap and application.
I am not a big fan of PIO caps and probably wouldn't put them in this circuit because I also think this circuit would not benefit from additional "roundness". But if you had different speakers or power amp or front end than me you might appreciate the additional roundness. So, it's quite subjective.
I've used the Russian PIO caps before without failure. I've seen some tube guitar amps that use them. They are Mil-spec. Should be quite durable I would think.
If a coupling capacitor can change the sound that much it must be "defective". It should be easy to measure its frequency transfer function and see it cuts the high end. I think it must be a psychological effect thinking of "paper in oil". I have measured a Jensen 10 uF pure alu foil PIO and it goes higher than I can measure (> 100 kHz). But what oil does of good things in a capacitor…...maybe dampen mechanical resonances?
One problem with physical large capacitors is that they can pickup noise. So they have to be far away from strong magnetic/electrical fields.
One problem with physical large capacitors is that they can pickup noise. So they have to be far away from strong magnetic/electrical fields.