Cascode Tube Mic Preamp: 12AX7/12AU7 Schematic

[88man]

I've been searching a long time for a tube mic preamp schematic for classical piano recording. I already have an Avalon AD2022 FET preamp, but I am looking to add a different palate of sound with a little tube sweetness, saturation, big tone, and a 3-D holographic sound WITHOUT changing the tonal character of my piano. Basically, lush without color - it exists but these preamps are over $4000. I've looked into Gyraf, Pultek, Boozhound, Hamptone, FiveFish, Sound Sculptor, Frontiernet, Aikido, etc. I am not sure if any of these are colored or transparent.

On the Jensen transformer page, I found an interesting tube mic preamp schematic that employs an input and output transformer and 12AX7/12AU7 topology - similar to the D. W. Fearn VT-2 Tube Preamp. Here is a schematic for a cascode tube mic preamp that I made by merging the input, preamp, and output stages that were scattered throughout their site.

The plate voltage of 250V needs to be very clean for this application. I've been saving a matched pair of Telefunken Smooth Plate, and RCA Black Plate tubes for such an application.

JENSEN TRANSFORMERS, INC. - APPLICATION SCHEMATICS
PS-1 High & Low Voltage Regulator Kit

Questions:
1. For a 250V supply, will the Aikido PS1 kit work well?...
2. Is the output coupling cap, C2, correct at 22uF?...
3. What is the current requirement for (2) 12AX7 and (2) 12AU7 tubes?... Is it around 23mA?...
4. Since I merged the schematic from 3 separate stages, are the part values correct?...
5. Jensen states, "Can be DC coupled to upper grid in output driver of JT-10K61-1M." Does that mean connecting the output to pins #7 and/or #2?... Not sure how this is done or what the benefits are for doing so?...

Any advice, info, discrepancies, etc. would be greatly appreciated, Thanks!

[Merlinb]

Originally Posted by 88man

Questions:
1. For a 250V supply, will the Aikido PS1 kit work well?...

Yes almost certainly.

2. Is the output coupling cap, C2, correct at 22uF?...

That really depends on the impedance of the output transformer, but you may well be able to use a smaller value (4.7uF say) since the output impedance of the SRPP is fairly high.

3. What is the current requirement for (2) 12AX7 and (2) 12AU7 tubes?... Is it around 23mA?...

About 1mA per 12AX7, and about 2.5mA per 12AU7 as shown (7mA total for four bottles then). However, you may want to run the 12AU7 at higher current, again depending on the impedance of the trabsformer.

5. Jensen states, "Can be DC coupled to upper grid in output driver of JT-10K61-1M." Does that mean connecting the output to pins #7 and/or #2?... Not sure how this is done or what the benefits are for doing so?...

They mean connecting pin 6 (anode) of the cascode to the 12BH7 White-cathode-follower stage on this datasheet: http://www.jensen-transformers.com/datashts/10k611m.pdf Basically replacing your SRPP with a White cathode follower.

[Allen Wright]

88Man,
The 12AX7 is not well suited for cascode operation, as it has very low transconductance, and may well distort.

Far more suitable tubes are high transconductance tubes such as the 6DJ8 or 5687.

Also for mic preamplification, transformers are not needed IMO. If you want to build a totally state of the art mic pre, then take a look at my RTP3D phono preamp on my site, we have made a mic pre using this topology and it's been a major hit with the top musicians who have tried it., such as Robert Plant, who used it on his recentish solo album.

Regards, Allen

[Jaap]

In my collection of schematics (no I have no picture of it to attach here) I have a design of a cascoded microphone amp from a Dutch magazine(Radio Bulletin, september 1958) with the ECC85 (6AQ8) tube.

I shall describe it:
inputtransformer, 470K from first grid to ground, first cathode has 1200 ohm to ground and capacitor 100 uF.
Second half of ecc85: 390K from grid to cathode, cathode of course on plate first half ecc85. 0,1 uF from grid to ground. 390K on plate, from top of that resistor to ground a 25 uF el.cap. and from the connection of the cap and the 390K a connection to B+ via a 33K resistor. On the plate of the top half ecc85 sits a 0,1 uF cap connected to a 47K pot. Underside of pot to ground, wiper is output. Both halves of ecc85 consume 0,7 mA, gain is 180 x , noise is 6 dB lower than EF86 tells the writer of the article.

[Michael Koster]

Originally Posted by 88man

I've been searching a long time for a tube mic preamp schematic for classical piano recording. I already have an Avalon AD2022 FET preamp, but I am looking to add a different palate of sound with a little tube sweetness, saturation, big tone, and a 3-D holographic sound WITHOUT changing the tonal character of my piano. Basically, lush without color - it exists but these preamps are over $4000. I've looked into Gyraf, Pultek, Boozhound, Hamptone, FiveFish, Sound Sculptor, Frontiernet, Aikido, etc. I am not sure if any of these are colored or transparent.

On the Jensen transformer page, I found an interesting tube mic preamp schematic that employs an input and output transformer and 12AX7/12AU7 topology - similar to the D. W. Fearn VT-2 Tube Preamp. Here is a schematic for a cascode tube mic preamp that I made by merging the input, preamp, and output stages that were scattered throughout their site.

The plate voltage of 250V needs to be very clean for this application. I've been saving a matched pair of Telefunken Smooth Plate, and RCA Black Plate tubes for such an application.

Questions:
1. For a 250V supply, will the Aikido PS1 kit work well?...
2. Is the output coupling cap, C2, correct at 22uF?...
3. What is the current requirement for (2) 12AX7 and (2) 12AU7 tubes?... Is it around 23mA?...
4. Since I merged the schematic from 3 separate stages, are the part values correct?...
5. Jensen states, "Can be DC coupled to upper grid in output driver of JT-10K61-1M." Does that mean connecting the output to pins #7 and/or #2?... Not sure how this is done or what the benefits are for doing so?...

Any advice, info, discrepancies, etc. would be greatly appreciated, Thanks!

Your Avalon has nice specs ;-)

Those designs you list are all very different and will give you different results. IMO the Gyraf is the most thought-out design for a general purpose mic pre. I like the way they handle the gain range setting. I haven't heard one though.

If the pre is flat it won't change the tonal character of your instrument.

The thing I want to mention though is that if you're doing a DIY mic pre you have a huge opportunity to clean up the design and improve the performance by simply not building it for every blasted mic in the world ;-)

The goal of being able to use the same pre for tube condenser mics, phantom FETs, and ribbon mics leads to serious compromise or at best a lot of complication to do it right. I've tried it and for DIY my answer is "more pres please".

If you can build it only for condenser mics for example you can lose 20 or even 30 db of that gain and make the pre quieter and more linear with more dynamic headroom and a lot simpler. You wouldn't need a step-up input necessarily but could use it for good advantage in a one-gain-stage design. Many condenser mics are designed to work into 1600 ohms (or other) and you can use this to optimize the noise ratio.

Another example. An ideal pre for a ribbon mic would look like a MC phono pre with a very special circuit or transformer input and 70-80db max gain but of course no EQ. And no phantom power or any unnecessary contacts in the input signalpath. Just like a MC input but with 30 feet of cable and XLRs ;-)

Signal transformers on micpres are A Good Thing. The isolation is very often essential on both input and sometimes output. It also allows impedance to be matched when needed e.g. a ribbon mic input stage.

Cascodes are good as are pentodes (Telefunken V72 uses an EF86-like tube) but I agree I would use 6922 or D3a, 417A, etc., low noise figure tube. Not that a good designer can't make a 12AX7 sound good, just would not be my choice for a micpre. I'm currently working on active loaded 2 stage triode but the wind may soon change.

It's all about dynamic range (beyond the basic signal path). Meaning that noise needs to be low and signal handling capability needs to be high. A hot tube condenser mic like a U67 can put put 2V peak-to-peak signal but needs input noise to be below 20 uV (only 100db actually) Anything inside this is a compromise, which is another reason trying to accomodate another 30db difference in mic levels optimally is a chore.

It's also useful to simulate the circuit using spice to do the gross level op point setting, cap sizing, frequency, noise, and distortion analysis.

Cheers,

Michael

[88man]

Merlinb, Allen, Jaap, Michael, thank you all for your fantastic responses. The consensus seems to evolve around the 6DJ8, D3, and 5687 topology for mic preamps. When trying out new equipment, I can make viable A/B comparisons between the recording and the source since I am recording my own piano. Besides the mics, it seems to me that the subjective component to the "sound" primarily comes from tubes and transformers within the audio chain.

The mics that I tend to use are the AKG C414B/XLS and Sennheiser MK8020. My experimentation with tube mics has been disastrous - nasal tone on my Steinway is the wrong tone for an acoustical instrument. I am still searching for a tonally neutral tube mic, but that's for another thread... The Brauner Valvet is on my short list!


Yes, Merlinb, I realized after posting that I forgot to include the 12BH7 stage in the cathode follower output stage in the schematic. Here is the entire schematic. I am glad at least the Aikido PS1 is adequate for a PSU.

By the way, here are the values for the output transformer:
Input impedance = 11.5k ohm
Output impedance = 133 ohm
DC resistance primary = 740 ohm
DC resistance secondary = 42 ohm

I am just curious if C2 coupling cap for the output transformer is right at 22uF, or if it can actually be lower?


Allen, I saw your impressive array of tube products. I am curious, what is the sonic signature of the RTP3D as a mic preamp? Do you have more details about the mic preamp in terms of cost, build, tone, transient response, bandwith, etc.


Michael, I looked at the Gyraf G9 again. Ironically, I did come across these comments in the summary while looking at the Gyraf more closely:
"The (Gyraf) G9 holds up against my favorite Jensen Mic-pre (a DIY project from the Jensen site)... My Jensen Mic-pre does sound good on just about anything so if the G9 wasn't as good as the Jensen I would have heard it immediately."
Kev's and Byron's G9

I noticed that the Gyraf G9 has 6 coupling caps in the signal path. I am not sure how that will affect the sound.

Michael, you're right, having a dedicated preamp for only large condenser mics would be ideal. Since I don't need more than 40dB gain, your suggestion of using a one-gain-stage design sounds great! Are there any schematics that you know of?...


In summary, with all that has been mentioned by everyone, I will try to keep an open mind about looking at other tube topologies besides the 12AX7. If anyone has a schematic of a high end tube mic preamp that sounds "lush without color" - please let me know!

Thank You All!

[88man]

The circuit claims 74dB gain. However, I only need about 40dB gain for my mics. Since the 12AX7 is not ideal for mic preamps,

Questions:
1. Would a 12AT7 be better than a 12AX7 first stage?... What part values would I have to change to use a 12AT7 for bias and plate resistors?
2. If not, how could I use the circuit to use a 6922?... I'll have to change the heater voltage too to 6.3V

Thanks

[Geek]

Not a beginners circuit, but it works.

Cheers!

[Jacques Merde]

Originally Posted by Geek

Not a beginners circuit, but it works.

Cheers!

Is that copyleft as in FSF Copyleft? Just curious.

[Geek]

Yes, only more under this specific priciple, since it relates to physical manifestations of designs, not just code:
Open source hardware - Wikipedia, the free encyclopedia

If attribution to me is maintained, go nuts

Cheers!