The pot is at the front if you didn't look at the schema. I've tested it well before I swapped the 1M for 50K. No difference I can hear what so ever, mind you it only connects to the soundcard anyway.
I've finished it today with only one change, the plate resistor to 100k.
DF96 mentioned there will be HF roll off, yes there is a little but it has a benefit to it as it filters some of the crappy harshness of MP3 format from the Net. Turning down the tone control covers too wide HF range, this seems to cover the higher band only. (Wish I could measure all this stuff!). I tried bypass caps which restored the HF (depending on value) but the sound got somewhat loose which I didn't like, probably because I'd need NFB with the cap.
Here is, it's pretty small but I made sure I've left space for 2-3 tubes in case I want to turn it in to something else in the future.
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No problem, more likely me just creating confusion. This simple one tube preamp has been quiet a headache to make actually. I guess that's a price I pay for making something out of norm. No reference I can look up to for ideas and not enough knowledge to calculate the non standard stuff.
I did a valve mixer using one 12ax7 for a mobile disco.
Both stages had the outputs at the anode.
I found it had no problem driving the 22K input impedance of the power amp.
The first stage was a mic stage.
The second stage took the output of the mic stage but also 2 stereo input channels and mixed them. Worked very well.
Both stages had the outputs at the anode.
I found it had no problem driving the 22K input impedance of the power amp.
The first stage was a mic stage.
The second stage took the output of the mic stage but also 2 stereo input channels and mixed them. Worked very well.
I did something similar few years ego for my home studio. 12AX7 and 12AU7 anode output. The quietest rec preamp I've ever had including commercial ones and no problem with loads either. Tech people say impedance MUST be matched, personally I've never had problems so not really sure how critical it is but if it works good....
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It all depends on what you mean by 'impedance matching'. Usually in audio a severe mismatch gves the best results!
However, driving low impedances (such as 22K) from a 12AX7 is a recipe for distortion. Two ways to avoid the problem:
1. keep signal levels very low
2. embrace the warmth of the distortion
However, driving low impedances (such as 22K) from a 12AX7 is a recipe for distortion. Two ways to avoid the problem:
1. keep signal levels very low
2. embrace the warmth of the distortion
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If you match (i.e. make equal) the load impedance to the source impedance you get maximum power transfer.
To transfer maximum voltage to the load, the load impedance should be high compared to the source impedance.
To transfer maximum current to the load, the load impedance should be low compared to the source impedance.
I often like my music at the near edge of distortion. That's why I like 10W tube amps, no need to go crazy loud to get my tone.
Ok I've finished it, it's all up and running on my desk now. This is the final with only change that I've added bypass cap because I've found the HF loss was not actually practical.
An externally hosted image should be here but it was not working when we last tested it.
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