Micpre of Graham
in [Design]
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From: Ban on 20 Mar 2007 11:34 The schematic http://rapidshare.com/files/21272377/mic_amp_2.jpg looks pretty simple, but it still needs work. I found a couple of gotchas: 1. When you switch on the phantom power the Vbe of the transistors gets reversed momentarily(+17V instead of -0.7V), degrading beta and Vos. This will slowly destroy the input devices. This happens always in normal operation with or without a mike. 2. The power supply rejection is very poor(-20dB) especially at higher frequencies. Here current sources might improve the situation. A lot of additional filtering is also needed. 3. When saturating the opamps will return to normal operation in a staggered way, creating spikes in the O/P signal. 4. The offset voltage varies with the gain, making it sensitive to variations in gain setting. I have attached a link to a commercial product, just to show that the art of making a good preamp is not *that* simple. http://rapidshare.com/files/21831341/mic_pre_02.png -- ciao Ban Apricale, Italy
From: Hawker on 20 Mar 2007 13:24 I'm always interested in looking at others mic pre-schematics. Just wondering who's the one you uploaded is. The fact that they are using 5534s and not say 2114s or even 5532s (which test a bit better than a 5534) makes me wonder about it. thanx Hawker On 3/20/2007 10:34 AM, The digits of Ban's hands composed the following: > The schematic > http://rapidshare.com/files/21272377/mic_amp_2.jpg > looks pretty simple, but it still needs work. I found a couple of gotchas: > 1. When you switch on the phantom power the Vbe of the transistors gets > reversed momentarily(+17V instead of -0.7V), degrading beta and Vos. This > will slowly destroy the input devices. This happens always in normal > operation with or without a mike. > 2. The power supply rejection is very poor(-20dB) especially at higher > frequencies. Here current sources might improve the situation. A lot of > additional filtering is also needed. > 3. When saturating the opamps will return to normal operation in a staggered > way, creating spikes in the O/P signal. > 4. The offset voltage varies with the gain, making it sensitive to > variations in gain setting. > I have attached a link to a commercial product, just to show that the art of > making a good preamp is not *that* simple. > http://rapidshare.com/files/21831341/mic_pre_02.png >
From: Ban on 20 Mar 2007 12:32 > > On 3/20/2007 10:34 AM, The digits of Ban's hands composed the > following: >> The schematic >> http://rapidshare.com/files/21272377/mic_amp_2.jpg >> looks pretty simple, but it still needs work. I found a couple of >> gotchas: 1. When you switch on the phantom power the Vbe of the >> transistors >> gets reversed momentarily(+17V instead of -0.7V), degrading beta and >> Vos. This will slowly destroy the input devices. This happens always >> in normal operation with or without a mike. >> 2. The power supply rejection is very poor(-20dB) especially at >> higher frequencies. Here current sources might improve the >> situation. A lot of additional filtering is also needed. >> 3. When saturating the opamps will return to normal operation in a >> staggered way, creating spikes in the O/P signal. >> 4. The offset voltage varies with the gain, making it sensitive to >> variations in gain setting. >> I have attached a link to a commercial product, just to show that >> the art of making a good preamp is not *that* simple. >> http://rapidshare.com/files/21831341/mic_pre_02.png Hawker wrote: > I'm always interested in looking at others mic pre-schematics. > Just wondering who's the one you uploaded is. The fact that they are > using 5534s and not say 2114s or even 5532s (which test a bit better > than a 5534) makes me wonder about it. > I think it is from Rane. Look at the offset adjust pins used for frequency compensation. The 5534A is IMHO better than the decompensated version(5532), less noise, higher slew rate and GBW, for gains above 3. -- ciao Ban Apricale, Italy
From: Eeyore on 20 Mar 2007 13:02 Ban wrote: > The schematic > http://rapidshare.com/files/21272377/mic_amp_2.jpg > looks pretty simple, but it still needs work. I found a couple of gotchas: > 1. When you switch on the phantom power the Vbe of the transistors gets > reversed momentarily(+17V instead of -0.7V), degrading beta and Vos. This > will slowly destroy the input devices. This happens always in normal > operation with or without a mike. The phantom power is separately switched on/off. It should as you correctly point out be of controlled rise/fall time. The same holds true for every mic amp I've ever seen btw. > 2. The power supply rejection is very poor(-20dB) especially at higher > frequencies. Here current sources might improve the situation. A lot of > additional filtering is also needed. So you say. Can you check that figure ? The only issue I see is mismatch of the 2 x 4k7 1% resistors and the 2 x 1k5 1% resistors. In practice these typically match within a batch to about 0.3%. I did simulate it btw. The power supply for that product has about 500uV of supply noise in the audio band and the 100Hz component is well down. > 3. When saturating the opamps will return to normal operation in a staggered > way, creating spikes in the O/P signal. I don't recall seeing that problem. Can you explain why you think that may happen ? > 4. The offset voltage varies with the gain, making it sensitive to > variations in gain setting. No, there's no DC gain wrt the offset voltage. You can twiddle it more or less noiselessly to your heart's content ! > I have attached a link to a commercial product, just to show that the art of > making a good preamp is not *that* simple. > http://rapidshare.com/files/21831341/mic_pre_02.png Would you like to run over the advantages of this ? It looks unnecessarily complicated to me to be honest. Graham
From: Eeyore on 20 Mar 2007 13:07
Ban wrote: > I have attached a link to a commercial product, just to show that the art of > making a good preamp is not *that* simple. > http://rapidshare.com/files/21831341/mic_pre_02.png Where do you get the 2k5 rev log pots ? The lowest value I've ever seen is 5k. I did a design with a dual gang gain pot btw. It worked rather well. I should perhaps point out that this design was rejected on cost grounds ! I hate to think what most mixer companies would say about your design. Tell you what, you cost yours and I'll cost mine ! Graham |