some non-linear tubes (someone said it) which sound super nice... 6BL7, 6BX7.
I am a happy camper with a transistor and 6DJ8 buggle boys holland made, for both DAC out and Phono. I will never be able to replace those tubes, which are now over 300$, I purchased them 15 years ago for almost 100$ each.
I use some nos 12au7 made in Japan in cathode follower to buffer the phono output.
I am a happy camper with a transistor and 6DJ8 buggle boys holland made, for both DAC out and Phono. I will never be able to replace those tubes, which are now over 300$, I purchased them 15 years ago for almost 100$ each.
I use some nos 12au7 made in Japan in cathode follower to buffer the phono output.
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I've got to raise my hand and disagree. TIM is a manifestation that there is a slew-rate limiting problem in the amp somewhere. It is perfectly possible to have a slew-rate limiting problem in an amp made of perfectly linear devices.
Slew-rate limiting is a different issue from linearity and demands different remedies.
Ummmm - RCA Tube Manual, I-V curves from mfg. datasheets, etc. I use my own eyes ! WRT TIM, it is only one form of non-linear distortion which cannot be completely addressed by negative feedback. There are a bunch of others that are poorly or incompletely understood and which tend to get worse as the device count goes up and the devices themselves act in a non-linear fashion. This is Amplifier Design 101. The sound of any particular amplifier is largely driven by the linearity of the amplifying device and the topology chosen to deal with it. The more linear the device and the simpler the design, the better, all else being equal.
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Slewing isn't a device limitation or flaw. It's just the inability to charge and discharge circuit capacitances fast enough, that is, to provide and sink current. An example would be the incorrect use of a high impedance valve like the type 12AX7/ECC83. Nothing at all wrong with the valve itself, properly used, but improper use gives it a bad rep. Driving big triodes means supplying current to charge and discharge Miller C, etc.
All good fortune,
Chris
All good fortune,
Chris
This is the statement I was correcting. Minimization of TIM is realized by making sure you don't have slewing issues, by making sure that each stage can drive the following stage to full amplitude without any slew-rate limiting.
See the following for a good explanation from Steve Bench to Ken Gilbert: slew rate limiting (bench) | ken-gilbert.com
Chris, are you talking about a simple circuit with a 12AX7 driving a high miller capacitance? Wouldn't that just lead to a simple rolloff as it is a low-pass filter? Wouldn't the slewing problems only creep up at higher drive levels if you don't have enough standing current in the 12AX7 stage and be independent of the rp of the valve?
Perzactly! My post wasn't for you so much as for others newer to the sport.
The classic definition of slew rate is when we drive an amplifier with a step (like from a square wave) and measure how fast the output can keep up, measured in volts per second. It's a limiting value that can (in the simplest possible case) also be described by:
Volts per Second equals (with a conversion constant) Charging Current divided by Capacitance to be Charged. Cool stuff, in and of itself, not?
And, for the 325,853rd time, I recommend to all amplifier builders, to get a passing familiarity with Op-Amp theory. It's simple (that's why it's useful) and helps to form useful models.
Our entire conception of reality is built on models. Finding good ones is how we progress and survive. My country is being taken over by bad models.
All good fortune, Chris
I like the statistician George Box's quote:
"All models are wrong, but some are useful."
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kinda reminds me of:
Dwight D. Eisenhower:
"In preparing for battle I have always found that plans are useless, but planning is indispensable."
I have 0 experience with choke loaded tubes, or transformer driven outputs etc.
Technically, I favor 1000% the humble capacitor to do this correctly.
However, the transformer comes in handy to help driver tube do a proper job and isolate it well, it is similar to choke loading it too.
A transformer can also be your phase splitter, this can be a great advantage if it is suitable to do so.
If you look for a 2 stage 845 amp, it is possible, but it will require probably some interstage transformer, the input might require a transformer too, then the feedback loop can be compromised.
There are a few things I would NOT do with a 845 tube:
1. have a b+ under 600V, no.
2. use it without feedback, no.
3. use a lower than 8k output transformer, no
4. drive it with a 6sn7 anode, no
I would drive it to extract all the power that it has
A powerful triode with local feedback and grid current sensors and regulators should be used there. A good coupling cap (high value) with something around 600V loading resistor and XXK on the anode should be adequate to drive the 845 to full power.
Use a 20W+ triode such as the 2A3 to drive the 845, seems most logical.
I would parallel 2 x 845 in S.E.T. but I don't know which transformer could take that.
Technically, I favor 1000% the humble capacitor to do this correctly.
However, the transformer comes in handy to help driver tube do a proper job and isolate it well, it is similar to choke loading it too.
A transformer can also be your phase splitter, this can be a great advantage if it is suitable to do so.
If you look for a 2 stage 845 amp, it is possible, but it will require probably some interstage transformer, the input might require a transformer too, then the feedback loop can be compromised.
There are a few things I would NOT do with a 845 tube:
1. have a b+ under 600V, no.
2. use it without feedback, no.
3. use a lower than 8k output transformer, no
4. drive it with a 6sn7 anode, no
I would drive it to extract all the power that it has
A powerful triode with local feedback and grid current sensors and regulators should be used there. A good coupling cap (high value) with something around 600V loading resistor and XXK on the anode should be adequate to drive the 845 to full power.
Use a 20W+ triode such as the 2A3 to drive the 845, seems most logical.
I would parallel 2 x 845 in S.E.T. but I don't know which transformer could take that.
Without NFB, yes, in almost all useful cases. Resistive loads and usual tube and stray capacitances.
Inside a NFB loop, if you drive hard past the low-pass corner, the input stage sees a square-wave and the output of the low-pass becomes a ramp-wave.
Tube amps are "usually" low-NFB and "usually" have enough raw bandwidth to cover real speech/music signals without too much strain.
High NFB can make a poor amp look "good" for small signals but larger signals will behave badly.
Wait and see if I am wrong. (*Someone* will pop-up with a counter-example.)
Sun Tzu put it "Victorious warriors win first and then go to war, while defeated warriors go to war first and then seek to win."
All good fortune,
Chris
Excellent. The sainted Flanders and Swann sang:
"Such models of friendship are precious and rare, though the friendship of models, (two beats) is not!"
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All good fortune,
Chris
I’m building one up at the moment, I expect to use output transformers from Monolith Magnetics although they will have to be bespoke - see here.
A transformer between the driver and the power triode may also be beneficial if you decide to go for class A2.
Best regards!
Negative feedback has nothing to do with slew rate limit. When it clips, it clips, and open loop gain goes to the drain. Try to correct frequency response by shaping it by a passive input filter instead of correcting by feedback, the result will be the same: slew rate limit, i.e. large fast signals will clip.
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