Mic Pre 12AY7 + 12AU7

[Emmathom]

Hi !
I recently built a 4-channel tube preamp with a lot of help from a forum member: @thor.zmt
The diagram is his design and I only followed his instructions.
I would like to take this opportunity to thank him once again because he was cool, patient and understanding regarding my low level in electronics.
He was also able to respect my specifications which was (if possible) to re-use certain components such as the input transformers (CAPI CA2622) the tubes (12AY7 & 12AU7) and of course my HV 230v-230v 130mA power transformer.

Then Thor invited me to present this creation because it has a particularity: it is not a copy or an adaptation of an existing circuit but an original creation.
He will be happy to answer questions that are a little too specific and which I would not be able to answer.

Below is the diagram. It's not a "conventional schematic" but it's mine and I like it like that since it is very understandable (I hope so).
Some more pictures will soon follow because I'm now finishing the pre... (should be on audio tests next week)
I bought a second hand out-of-order IMG STA500 amplifier for 20€ to use its box because I needed a rack mount (and a new rack box costs at leat 70-80€ in 2U)

I must inform that I don't own an oscilloscope nor a frequency generator so no possibilities to make precises measurements yet. I'm about to get this equipement soon...

For information, this pre had been first designed by members of another forum, much more focused on hi-fi but it did not give me complete satisfaction in terms of "sound color". That's the main reason why I decided to rebuild it completly.

This last version sounds very good and offers much enough gain (for static mics & dynamic ones).
The Tx In are CAPI CA2622. Tubes 12AY7 & 12AU7. Output by Mosfet (IRF710).
One switch bybass or not the 12AY7 cathode R (activated by relay).
One switch 3 positions offers : no PAD / -12dB PAD / - 25dB PAD
HV is provide by a 230v-230v 130mA Tr / 48v by a 230v-2*24v 16VA Tr / heater by a 230v-12v 1,3A Tr

 

[Emmathom]

I continue with the power supplies whose diagrams are attached.
In the first version the output was by a Jfets arragement and required a dedicated +15v power supply.
The LEDs were then powered by this +15v with R in series.
Since the Jfets were replaced by a Mosfet output I lowered the voltage of this power supply to +12v in order to power the different LEDs as well as the 4 relays which switch the bypass capacitor

 

[Emmathom]

... and a schematic overview which helps me (as a reminder) where to place and which values for the components & wiring...

 

[rock soderstrom]

Nice project! I also like to switch the cathode capacitor of V1 in this way, which results in interesting variations for the sound of the preamp.

Extra karma points for recycling old cases looks good! Real DIY.

What is the range of possible output voltages of the HT power supply with the 20K potentiometer/trimmer?

 

[Emmathom]

What is the range of possible output voltages of the HT power supply with the 20K potentiometer/trimmer?

I discovered this DN3545 depletion mosfet in an hifi forum. This assembly (by Yves Monmagnon cf.) is interesting because it can handlle 450v max. with an 200mA IDSS.
So it acts as "a remote" for the power mosfets and you choose your volatge as 1KΩ = 1v.
So I put a fixed 270KΩ + a RV 50KΩ to adjust the voltage > 270KΩ + 20KΩ gives me 290v !

The combination of the 100KΩ (to the power mosfet gate) + the 10uF acts as a soft-start as :
(1000v per second - per uF) x by the 100KΩ (RxC)
So in my case, it takes almost 100 sec. to reach 290v but it's not linear : a kind of log curve as you get 60-90v in a couple of seconds and then the rest comes slowly

http://www.dissident-audio.com/RegulHT/Regul.html

 

[Brian Roth]

@Emmathom Interesting project! I just took a look at the schematic in your first post. Any reason you chose 4k7 for the phantom powering resistors? 6k8 is considered the "standard" value.

Anyway, keep up the good work!

Bri

 

[Emmathom]

@Emmathom Interesting project! I just took a look at the schematic in your first post. Any reason you chose 4k7 for the phantom powering resistors? 6k8 is considered the "standard" value.

Anyway, keep up the good work!

Bri

Thank You !

@thor.zmt could explain this arrangement better than me... but it is to filter the 48v (1KΩ + 160uF is recommended for -3dB•1Hz) before sending it to the microphones. I put 68uF because I had them in stock (so -3dB•2,36Hz) and it's such a deal to change them as I had to "cut" a special board for these 48v switches (see attached). In the future I would put 220uF • 63v to be "safe"

And so we must calculate like this:
1KΩ + (4.7KΩ // 4.7KΩ) = 3.350KΩ*2 (pins 2&3) = 6.7KΩ
I hope this is right...

 

[thor.zmt]

Any reason you chose 4k7 for the phantom powering resistors? 6k8 is considered the "standard" value.

The official P48 standard is 6.8k from 48V, giving around 10.6mA available to the microphone if the microphone "pulls down" to 12V (which is a sensible operating voltage at the lower end).

It is common practice (and a good idea), especially when using 48V SMPS, to include an RC filter in the 48V line to suppress noise.

The value of the Resistor in this RC filter needs to be accounted for. We cannot increase the 48V, as doing so will push "low current" microphones out of spec. Instead we need to lower the actual Phantom Power resistors to account for the difference, or we risk "starving" high current consumption microphones.

@Emmathom gave the calculation.

Now, using 4.7k+4.7k will lower the loading on the input. However the main load is from the input transformer. We commonly see something around 1...2kOhm. Compared to that effect from our resistors is not material.

Now, we could make a regulator that senses the 48V after the resistors but completes the AC feedback loop (which determines noise) before this resistor and thus compensate the resistor, but it's a little less trivial.

Another option is to replace the resistor with large value low DCR choke and "Wenzel finesse" type noise killer may be employed using the DCR of the Choke as "ballast", this means using a Sziklai pair with a P-Channel, low noise, high transconductance Mosfet combined with a low(est) noise NPN transistor to get enough noise rejection.

I felt that to simply adjust the resistor values was the easiest for the OP and will be for most people.

Thor

 

[Brian Roth]

Well.....using a random SMPS vs a linear....everyone is on their own. Have fun!

Bri

 

[thor.zmt]

Well.....using a random SMPS vs a linear....everyone is on their own.

A lot of people promote exactly that, use a random SMPS for LED's.

I just finished a project with a customer for an Audio Power system, that starts with a "random" 350VA/24V SMPS and creates 12V Aux, +/-15...21.5V adjustable and +48V.

Using Multi-phase clock synchronised 750khz switchers the system has ~20mV P-P ripple at 3/6MHz and design output current BEFORE the LC filters on the output.

And due to the number of parallel converters audio band noise is also very low, as we get noise down on the general noise levels of the bandgap references, so we get less than most 3-Pin regulators..

That cleans up all noise from the original SMPS and creates a high efficiency "Audio Grade" supply for large mixing desks that can take on pretty much any Linear Supply and don't get birdies from interference between different switching frequencies of multiple SMPS.

Sadly, such systems are way beyond the capabilities of the average hobbyist, so I normally recommend linear supplies where possible and to use heavy LC/RC filtering over electronic power supplies, as they are less likely to spontaneously explode than 3-legged fuses.

Thor

 

[Emmathom]

About SMPS :
One of my friend & I use them much, specially in Mosfet power amps.
1x250w • 36v per channel for a 2x100w • 8Ω and no differences at all when listening (we have exactly the same amplifiers - same boards & components - with both linear PSU & SMPS)
You have to buy quality, of course, but It's a bit cheaper, much lighter (in weight) and as good as any linear PSU
That's my point of view...

About my pre's :
- I've got a "Mix pre" 4 chanels with exactly the same circuit for 2 tubes channels + 2 Jfets channels. In this particulary pre, the 12,6v heater (300mA) and the 48v (4 channels) are provided by 2 different SMPS from Meanwell (good brand). I've been recording with this pre for hours and never encoutered any trouble (12 hours a day for at least a week)

- in the Quad exposed here, all the voltages are provided by conventional linear PSU's...

Besides that, my recommandation is to use a soft-start (espescially when high current like a power amplifier) because these SMPS seem to not appreciate strong current draws (when capacitors are empty...) But anyway a soft-start is a good habbit when high currents (from several A to...) with huge stocking capacitors are used...

 

[Emmathom]

Hey !
I've finished today the Quad and did some audio tests (only to check what's going on)
- It works fine on the 4 channels
- the cathode bypass by relay too
- the Pad : -12dB is fine, but -25dB is too much and it does not offer interest as well > I think I will change the 2,2KΩ resistor (in // with 1MΩ) for a 10KΩ to get -18dB in full position

My firts impressions : the sound is nice ! balanced and not so much "H2 colored" when K is unbyp. / there's more than enough gain for static mics and even for dynamic or ribbon with K byp. : I've tried with a SM58 but I ain't got a ribbon...

So I'm very pleased with this pre circuit! It was quiet a lot of job especially about wiring all these switches (and of course the ptp main boards)
Thank You +++ @thor.zmt !
More picts soon...

 

[Emmathom]

I may deviate a but from the original topic but I tried 2 mics with this pre today : my t.bone SC1100 modified (with the great help of @micolas ) and my brand new Stam Audio SA47 fet
I prefer using the term "modified" for the t.bone since I just changed the capas. for better ones and rebias the K170.

The result : in term of gain there is a huge difference !!! The t.bone delivers +18dB (I would say so but I've not got instruments to measure it) compare to the Stam Audio... and I'm a bit perplexe because the SA costs 700€ (or so with taxes) and the t.bone 110€ (with few new components).
I post here the schematic of my modified SC1100 with real voltages : maybe some values are "too much" and bias the mic "way too hot" ?
I don't understand this huge diff. of gain between a professionnal & very appreciated mic and a cheap one...