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Wow bvtrinh! That's fantastic looking amp!
I think input transformers have been discussed in this thread before, but I'm not quite sure I fully understand the concept. Would a 1:4 ratio quadruple the input voltage at the expense of current? Would this increased voltage allow a lower gain pre-amp to effectively drive the MoFo?
I'm currently building a battery powered tube pre-amp similar to Bottlehead's Quickie. It was mentioned earlier that this pre-amp would not have enough gain to drive the MoFo. Would adding an input transformer make it possible to use the Quickie with the MoFo?
If you guys can shed some light on this, I'd be very grateful!
I think input transformers have been discussed in this thread before, but I'm not quite sure I fully understand the concept. Would a 1:4 ratio quadruple the input voltage at the expense of current? Would this increased voltage allow a lower gain pre-amp to effectively drive the MoFo?
I'm currently building a battery powered tube pre-amp similar to Bottlehead's Quickie. It was mentioned earlier that this pre-amp would not have enough gain to drive the MoFo. Would adding an input transformer make it possible to use the Quickie with the MoFo?
If you guys can shed some light on this, I'd be very grateful!
Thank you Jenghis! I’m sure there are plenty of people who can answer your question about the input transformer better than I can but yes, I used the input transformer for two reasons: 1) so I could use the same preamp with the MoFo and 2) to convert single ended to balanced since mine is actually two MoFos bridged per side. The only thing to keep in mind is the preamp output impedance. The MoFo has a 50k input impedance so it worked out fine.
Bao
Bao
Thanks Bao, I didn't think about the input impedance. My preamp has a rather low output impedance of 2K ohms, so I don't think impedance matching will be an issue.
EDCOR - XSM Series
I wonder if one of these Edcor line matching transformers might work as an input transformer?
EDCOR - XSM Series
I wonder if one of these Edcor line matching transformers might work as an input transformer?
A 4:1 step up transformer will increase the output Z by a 16:1 ratio.
Your preamp will have a 32K output Z.
If I needed a step up transformer with a tube preamp, I would put a jfet buffer in front of the transformer. In theory, a transformer has zero distortion when driven from a zero ohm source impedance. While I doubt that, the point that transformers do better with low driving impedance's is valid.
HTH
Your preamp will have a 32K output Z.
If I needed a step up transformer with a tube preamp, I would put a jfet buffer in front of the transformer. In theory, a transformer has zero distortion when driven from a zero ohm source impedance. While I doubt that, the point that transformers do better with low driving impedance's is valid.
HTH
To put things in perspective, I’m using the Bruno Putzey’s Balanced Preamp which has an output impedance of less than 50 ohms. It works very well with a step-up transformer because even with the Lundahls LL1674, the output impedance is still less than 800 ohms. I believe there was another member in this thread who used the Edcor with success.
Post 1615 https://www.diyaudio.com/forums/pass-labs/313649-build-mofo-162.html#post5623876.
His source output impedance is 32 ohms though.
Bao
Post 1615 https://www.diyaudio.com/forums/pass-labs/313649-build-mofo-162.html#post5623876.
His source output impedance is 32 ohms though.
Bao
Ah, that makes sense. I was thinking in terms of an electric guitar, whose output impedance is something like 100K.
That's an interesting idea, would that be something like a simple common drain buffer?
Ah, now I understand. A low output impedance preamp is best for use with an input transformer.
Thanks for enlightening me guys!
Yes. A buffer like we have been discussing for the B1 or LX mini (among others) could be easily adapted.
Ideally you want drive Z much lower than the transformer rating and the output load much larger than transformer rating.
For example, a 600:10K transformer would be happy being driven with a 60 Ohm output and loaded by a 50K load.
This idea sounds like a single ended version of the First Watt M2. Would it be possible to adapt one of the daughter boards used in that design for the MoFo?
Well, that would basically be Zen Mod's "Iron Pre"
The boards should be available this summer, here's a photo of the first revision -
What's wrong with the kiss,boy ?
The M2x daughterboards are buffers, the transformer on M2x is on the backplane. So it's basically the same idea, but the Iron Pre will be much more appropriate for preamp duties.
Guys I need some advice please.
I'm thinking of replacing the Meanwell switcher on the MoFo with a Linear one, as the build-in cooling fan on the Meanwell starts kicking in after 10 mins of running and will come on every 3-4 mins after that, it sounds like a hair dryer, very annoying.
I have read somewhere in this thread Michael had build a version with 36V @4A with one-fifth of distortion(yet to figure out this 1/5 of distortion means 20% THD distortion or 1/5 lower THD distortion?) I happen to have a whole stacks of 2x25V 300VA toros laying around and wondering if I can use those instead. I'm not going to bias it @4A though as the my CH-4 inductors are rated 4A only, I'll keep it @2.5-3A
The heatsinks I got are Conrad 150x300. When I run the big Mofo with 2.5A, the heatsinks are barely warm, my laser thermometer shows 37C on the MOSFET body after 30mins, so those heatsinks may be able to handle 90W of power if I run 36V@2.5A or maybe a bit more.
I'm a bit undecided on the higher voltage/lower bias Vs lower voltage/higher bias. Could someone shed some light on that please.
Thanks.
I'm thinking of replacing the Meanwell switcher on the MoFo with a Linear one, as the build-in cooling fan on the Meanwell starts kicking in after 10 mins of running and will come on every 3-4 mins after that, it sounds like a hair dryer, very annoying.
I have read somewhere in this thread Michael had build a version with 36V @4A with one-fifth of distortion(yet to figure out this 1/5 of distortion means 20% THD distortion or 1/5 lower THD distortion?) I happen to have a whole stacks of 2x25V 300VA toros laying around and wondering if I can use those instead. I'm not going to bias it @4A though as the my CH-4 inductors are rated 4A only, I'll keep it @2.5-3A
The heatsinks I got are Conrad 150x300. When I run the big Mofo with 2.5A, the heatsinks are barely warm, my laser thermometer shows 37C on the MOSFET body after 30mins, so those heatsinks may be able to handle 90W of power if I run 36V@2.5A or maybe a bit more.
I'm a bit undecided on the higher voltage/lower bias Vs lower voltage/higher bias. Could someone shed some light on that please.
Thanks.
In principle, the "optimum" V/I is equal to your load impedance.
Actually you can allow another Ohm for MOSFET drop.
The example 36V/4A works out to 9 Ohms. Take-off an Ohm for MOSFET drop, should be 8 Ohm load.
That's for decent power economy. If you are cold, and electric heat is cheap there, you can run higher current with some drop of distortion. (As you say, your choke-choice may suggest otherwise.)
If you load were 16 Ohms you would be fine with 36V 2A. If you like heat, 36V 3A or 36V 4A (LOT of waste heat here).
If you face 4 Ohm loads, you really want lower voltage and higher current. 24V 5A might be a good goal for 120W dissipation (30W-50W audio output depending on the THD you call "enough").
All of this gets real screwy when you drive Loudspeakers, which have impedance that varies all over the place. Back in the 1930s we kinda-agreed that the lowest valley of the impedance curve was the nominal impedance, because that is where the amp will work the hardest. Many hi-fi speaker builders didn't get the memo, or think it does not apply. Occasionally tricky crossovers cause massive impedance dips in narrow bands. While push-pull amps will often pull such loads without rude noises, a single-ended amp working hard may splatt these bands prematurely.
All that said..... your proposed ~~100 Watt dissipation is a pretty big amp. And someone says the First Watt is the most important, which a 100Wdiss amp will do with ease even when not optimized for the fortieth Watt.
Thank you PRR, great explanation yet entertaining. I may give it a try. Like I said I have more 300VA donuts than know what to do with them, and I just don't wanna spend any more coins on another one.
Another question, currently I have 35V output cap on the big MoFo, do I need to change it to higher voltage to accommodate the higher B+?
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