If you load with a minimim load the sec. of a 1:10 trafo, p.e. 2kohm ( I don't know your value) you have reflected on primary 20 ohm that can be enough for you ribbon.
The same value you can reach with different trafos because it is a impedance translator; you have only to calculate the R on secondary.
The value I give you are quite good for your purpose; you can go down to 10 kohm, just to check.
Bye
Walter
The same value you can reach with different trafos because it is a impedance translator; you have only to calculate the R on secondary.
The value I give you are quite good for your purpose; you can go down to 10 kohm, just to check.
Bye
Walter
Yeah. Checkup with rocking everything by a wooden stick revealed cold solder. Also I've replaced one cap, just in case.
But it appears that the reason for the weird "quietness" is my sound card that is looks like deaf now.
Probably a signal from preamp was so strong that it has burnt the cards input channels. One is completely deaf, anther barely hears. Not sure if it is fixable.
Otherwise the preamp works well. And audibly produces less hiss than my Behringer Xenix1202 mixer mic XLR input.
But it appears that the reason for the weird "quietness" is my sound card that is looks like deaf now.
Probably a signal from preamp was so strong that it has burnt the cards input channels.
One is completely deaf, anther barely hears. Not sure if it is fixable.Otherwise the preamp works well. And audibly produces less hiss than my Behringer Xenix1202 mixer mic XLR input.
It is not that early. Most of ribbon mics are up to 15kHz. Mine is (according to calculations) should be somewhere up to 20kHz.
It is may be a good idea, though THD is one order more but if it will reduce noise why not. I'll give it a try.
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Hi,
Surely not just attributable to Miller capacitance on its own?
Is such an early roll-off acceptable for a mic?
Why not use a cascode as the first stage? That would at least not suffer too much from Miller effect and, most importantly lift the input signal above flicker noise levels (hopefully).
Ciao,
LTSpice simulation for cascode shows poor noise and freq response rolloff above 10kHz.
Measured V~ on output (with 4.6k load) during buzz when input tube grid touched . V was up to 140V~ ! Wow. That is brutal. It is good that mixer preamp has a protection.
I've added two 2.7V Zeners back-to-back across output, and 100R ballast resistor. Voltage was not higher than ~3V when I rubbed mic.
I do not hear any impact on normal sound.
Added two bright (4500mCd) red LEDs in between Zenners, that indicates overload. Cannot measure impact of the protection on sound because do not have good sound card. But I do not hear any audible impact for signals of normal level. And high level is distorted anyway.
Here is my current schematic.
Here is my current schematic.
Attachments
New addition to overflow protection - magic eye indicator.
I've added this 6E2ÏÔʾÇý¶¯°å
indicator to output. It serves dual purpose: helps to set proper signal level, and this tube looks really magic . I was going to add a line of LEDs, but then decided to make it all tubes. At least how it looks externally.
I hope, 9014 across output signal path will not spoil sound too much.
Or should I make it switchable?
I've added this 6E2ÏÔʾÇý¶¯°å
indicator to output. It serves dual purpose: helps to set proper signal level, and this tube looks really magic
. I was going to add a line of LEDs, but then decided to make it all tubes. At least how it looks externally.I hope, 9014 across output signal path will not spoil sound too much.
Or should I make it switchable?
In attempts to improve noise level, I've measured the preamp's "sound" when it is totally un-powered (by switches) and still plugged into wall mains outlet.
There is some buzz (60Hz *N) that comes most likely from Earth loop.
I've described the issue here http://www.diyaudio.com/forums/power-supplies/257066-weird-buzz-when-psu-turned-off.html
But still do not have any solution. Could you please take a look.
I tried to completely disconnect Audio Ground (AG) and chassis - it creates too much noises and radio.
I tried to put R 20 Ohm between chassis and AG - buzz is same as directly connected.
Any other ideas?
Chassis is alum box, and it MUST be earthed.
There is some buzz (60Hz *N) that comes most likely from Earth loop.
I've described the issue here http://www.diyaudio.com/forums/power-supplies/257066-weird-buzz-when-psu-turned-off.html
But still do not have any solution. Could you please take a look.
I tried to completely disconnect Audio Ground (AG) and chassis - it creates too much noises and radio.
I tried to put R 20 Ohm between chassis and AG - buzz is same as directly connected.
Any other ideas?
Chassis is alum box, and it MUST be earthed.
I'm not the exert on transformers. But here is what I do....
First decide on a budget. Am I building a "best at any cost" device or maybe I have a budget?
Then I select the BEST quality transformers my budget allows. I don't think yet about a schematic. The transformer will cost about 15 times more than the tubes and the transformers will determine the sound to a greater degree than the tubes. The two audio transformers will dominate the design. SO pick those first. Transformers can run from a low of about $35 to up to $125 each and you ned two of them (plus a power supply transformer) The tubes are about $15 each and each one has two sections, or $7.50 per triode. When you look at cost and audio quality we are building not a "tube preamplifier" but a "transformer amplifier that happens to have some tubes inside." The main part of the amp by any way you want to measure (cost, weight, importance,...) is the magnetics. The next most important and costly part is the chassis.
Next I follow the transformer manufacturer's reference design. At least use it as a start.
After which transformers you select the next most important thing might be the chassis and physical construction and grounding plan.
See where I'm going? DOn't just slap on a transformer to an existing design. But if you like the output impedance of the existing design a 1:1 transformer will not change it.
But be warned that unshielded transformers can pick up noise from the environment.
It is entirely possible you get some hum when the mic pre is NOT powered. The tubes are open circuit so your output looks like a 1Meg resistor in parallel with some back to back diodes. It is not surprising this might pick up some hum.
To minimise hum in your build I recommend you read this:
http://www.ianbell.ukfsn.org/EzTubeMixer/docs/EzTubeMixer/SimpleMixer/grounding101v2.pdf
Cheers
Ian
Thank you guys.
I'll try first with cheap 1:1 (600R:600R) transformer I have, I know it will cut off LF and HF, but it should remove buzz. If it works I'll buy a good transformer.
Earth buzz is still puzzling me. I suspect that Earth of my computer, that is connected via UPS, is somehow different (has buzzing potential) with Earth of preamp, and if I connect both, it will still introduce buzz even with output transformer. I doubt that E-MU 1212 has input transformers, I assume it has AG connected with Earth.
Are my suspicions correct?
Should I break that loop by connecting screen of output TRS cable only on one side?
Like detaching Pin 1 from Earth on output preamp TRS?
Also somewhere I read that nothing else should be attached to Earth chassis bolt.
Ian proposes to make it the Audio Ground as well. Will it be legal?
From other sources also is advised to make the Audio Ground at input, because it is most sensitive place, any hum here will be then amplified. I've made this way, but still have some buzz. Could you comment?
I'll try first with cheap 1:1 (600R:600R) transformer I have, I know it will cut off LF and HF, but it should remove buzz. If it works I'll buy a good transformer.
Earth buzz is still puzzling me. I suspect that Earth of my computer, that is connected via UPS, is somehow different (has buzzing potential) with Earth of preamp, and if I connect both, it will still introduce buzz even with output transformer. I doubt that E-MU 1212 has input transformers, I assume it has AG connected with Earth.
Are my suspicions correct?
Should I break that loop by connecting screen of output TRS cable only on one side?
Like detaching Pin 1 from Earth on output preamp TRS?
Also somewhere I read that nothing else should be attached to Earth chassis bolt.
Ian proposes to make it the Audio Ground as well. Will it be legal?
From other sources also is advised to make the Audio Ground at input, because it is most sensitive place, any hum here will be then amplified. I've made this way, but still have some buzz. Could you comment?
OK. In which case you really need to read a book on elementary electronics before going any further. Horowitz and Hill is the standard text book for electronics and fortunately you can download a copy of one of the older editions for free. Google is your friend.
Cheers
Ian
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