Hello George and All,
I too have done a fair amount of experimenting with the 12AU7 and variants.
Using a Keysight variable voltage power supply and an Audio Precision analyzer I found that you can zero in on a supply voltage and bias to find a sweet spot to minimize distortion. You can nearly disappear the 3rd and higher harmonics.
Then put the 12AU7 into a Shunt Regulated Push Pull circuit and adjust the load impedance and the 2nd Harmonic cancels.
Lots of fun playing with tubes
Thanks DT
Neat. I don't have an AP, but I can put my scope in FFT mode. Haven't done any kind of hooking up of the instruments yet as I was mostly experimenting with topology, but that's next for sure.
I have yet to try the SRPP configuration, and some more circuit variations.
I find them more fun than SS too.
User name checks out, DT
You can use any digital audio interface with REW software for a nice analyzer.
more here:
https://www.diyaudio.com/community/threads/howto-distortion-measurements-with-rew.338511/
Here is FFT of the Alpha 20 Class A amp at 2.83vrms using a $99 Focusrite Solo USB audio interface and REW.
more here:
https://www.diyaudio.com/community/threads/howto-distortion-measurements-with-rew.338511/
Here is FFT of the Alpha 20 Class A amp at 2.83vrms using a $99 Focusrite Solo USB audio interface and REW.
This plot does not do anything for me.
How was it measured?
What was measured?
What is dBFS? Is it calibrated to something? Is it dBV? Is it SPL?
It is just hanging out there.
Thanks DT
Yes! I really like R.E.W., been using it for a few years now, as well as Audacity which has a nifty internal Spectrum plot. R.E.W. calibration from the soundcard is really super useful for better measurements here. It gets really better with each release.
I have three ADCs I can use: an old M-Audio which caps at 96kHz, an Apogee Ensemble and a newish Behringer UMC-202HD. The latter two can both do 192kHz (less than half that for actual Nyquist response, esp. the Behringer, but that should cover good cases). The Apogee has an internal SMPS though. I thought the Behringer had quite good isolation from USB bus power since when I used the DIY USB isolation connector and my DIY Linear Regulated PSU, I couldn't detect any audible change in sound quality.
The Behringer can be improved upon ADC-side and a member here did some great work to enhance this.
A great ADC + R.E.W., and you have yourself a setup that rivals some older AP if I'm not mistaken.
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I’m just showing you an example of what it can do. Go to the thread to the Howto use REW thread for distortion measurements to find out more.
That was an 8ohm dummy load measured at output of the Alpha amp using a basic resistor divider to keep signals below clipping of the front end mic XLR input. I think I used internal REW frequency source as oscillator. Later on I use external Akitika 2ppm of Victor’s ppb oscillator. The scale is in the graph title: 2.87vrms is -20dBfs.
That was an 8ohm dummy load measured at output of the Alpha amp using a basic resistor divider to keep signals below clipping of the front end mic XLR input. I think I used internal REW frequency source as oscillator. Later on I use external Akitika 2ppm of Victor’s ppb oscillator. The scale is in the graph title: 2.87vrms is -20dBfs.
I wouldn't let that stop you from trying it in your setup and explore its features.
R.E.W. is absolutely stellar analysis software.
I actually only found out about it in a Home Theater forum when I was at the very beginning of my Audio DIY pursuit: I needed a way to correct my room response in an acoustic, organic way (no DSP). I made custom acoustic panels from there.
And that's precisely the original purpose of R.E.W.: measure and correct room acoustics.
Getting serious(ly hot):
Small half op amp socket with leads for the breadboard, and small heatsink with isolated mount and heat conducting separator.
Will allow for longer testing on the breadboard with proper biasing and also allow easy swap of transistors. The heatsink can rest on the desk or on a larger piece of aluminium if necessary.
Small half op amp socket with leads for the breadboard, and small heatsink with isolated mount and heat conducting separator.
Will allow for longer testing on the breadboard with proper biasing and also allow easy swap of transistors. The heatsink can rest on the desk or on a larger piece of aluminium if necessary.
On the Virtual Bench Tonight:
Found something odd in LTSpice: I compared two models for 12AU7 and I get surprisingly different results:
Puzzling stuff.
I have also pretty much given up on simulating the non-conventional tube circuits as these are never modeled in the first place.
As an aside, delivery for the new video Receiver never happened in time as the delivery man didn't do his job (no knock on the door, and not even a note on the door), lied by putting a delivery note online saying we were 'closed because of COVID'. Besides, the customer service Rep. assured us we would get delivery the next day, which didn't happen either, and hence we asked for a refund.
We get a knock on the door today, so several days later, but we gave him the package back to return. Purolator sucks.
Found something odd in LTSpice: I compared two models for 12AU7 and I get surprisingly different results:
- The Vpp output is different, by a multiplier of 2, so 2mV vs 4mV
- The THD is different by an order of magnitude - 147ppm vs1786 ppm
- To keep the Output stage Source pin at similar voltages for no large distortion at output, I need to set the Plate Resistors to quite different values: 2.2k vs 5.7k
Puzzling stuff.
I have also pretty much given up on simulating the non-conventional tube circuits as these are never modeled in the first place.
As an aside, delivery for the new video Receiver never happened in time as the delivery man didn't do his job (no knock on the door, and not even a note on the door), lied by putting a delivery note online saying we were 'closed because of COVID'. Besides, the customer service Rep. assured us we would get delivery the next day, which didn't happen either, and hence we asked for a refund.
We get a knock on the door today, so several days later, but we gave him the package back to return. Purolator sucks.
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I think tube models are relatively inaccurate due to manufacturing variability. Ultimately, we had to make a test PCB, order the tubes and test to check the setpoints.
On the bench today is my old miniDSP 2x4 setup with two TPA3116 amps and a PCA preamp. Will be using it to test out a new subwoofer design I made that has 4x 10in drivers in a ring topology.
On the bench today is my old miniDSP 2x4 setup with two TPA3116 amps and a PCA preamp. Will be using it to test out a new subwoofer design I made that has 4x 10in drivers in a ring topology.
I guess I'll have to attempt measuring tube responses myself for the alternate circuits. More work than fun...
Intriguing topology. I have a smaller sub planned with some ideas around the cabinet and configuration of two drivers. I salvaged a few drivers from discarded subs. Quite a long way off of doing that as I need some more experiments for the hybrid headphone amp and the tube pre-amp.
Still, I run a few things in my mind in preparation: cross-over + more MOSFET tests for the sub amp.
Intriguing topology. I have a smaller sub planned with some ideas around the cabinet and configuration of two drivers. I salvaged a few drivers from discarded subs. Quite a long way off of doing that as I need some more experiments for the hybrid headphone amp and the tube pre-amp.
Still, I run a few things in my mind in preparation: cross-over + more MOSFET tests for the sub amp.
It’s a commercial design I have been working on with a team of partners in Califomia for a number of years now. The company is Mobius Acoustics - we were at the last in person Burning Amp event. This is an early prototype. Once it is released we can give more details about the internals.
more info on Mobius Acoustics here:
https://instagram.com/mobius_acoustics?utm_medium=copy_link
more info on Mobius Acoustics here:
https://instagram.com/mobius_acoustics?utm_medium=copy_link
It’s a new horn topology that Anthony Bisset and I came up with.
Here was our display at Burning Amp. I was not able to make it but others from Mobius were there.
On the bench tonight is the top speaker from this setup. I am developing a passive crossover for it. We can save a DSP active amp channel this way.
Initial simulations are good. Will make some Wago connector P2P and try it out soon.
Pulling out the crossover development kit. It’s grown a bit and has quite the assortment nowadays. The copper and iron adds up quick.
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