Mic Preamp with unshielded input transformers.

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

Heikki

Well-known member
Joined
Jul 31, 2008
Messages
387
Location
Finland
Here's a mic preamp with low noise and hum even without shielded input transformers. DC-DC converters are used in the power supply to make B+ for the tubes and +/- 15V for the op amps. Input transformers are Neutrik NTE10/3.

Schematic
Transformers and and 1:3 / 1:10 switch not shown on the schematic.

Clip indicator schematic

Noise at 61 dB gain, 200 ohm resistor across input XLR pins 2 & 3

I think that's pretty good with unshielded transformers since hum doesn't really ruin the noise figure. Most commercial tube preamp specs don't say anything about noise, so I can only assume this is better than most.

Here's a song we recorded yesterday using the preamp.
 

Attachments

  • micpre1.jpg
    micpre1.jpg
    9.6 MB · Views: 2
  • micpreandbasspre.jpg
    micpreandbasspre.jpg
    11.9 MB · Views: 1
Last edited:

Bo Deadly

Well-known member
Joined
Dec 22, 2015
Messages
3,078
Location
New Jersey, USA
Cool.

You should try to figure out where the hum is coming from. Not that it's a problem for a tube rig but it would be good to know the source.

It seems to me with it being entirely SMPS and the input is just a 200 ohm resistor and the output is fully balanced, it should be completely hum-free and year clearly that is mains hum.

I can only conclude that either the usual ambient electromagnetic radiation is getting pickup by the transformer or the high impedance / high gain bits or there is a problem with earth grounding or there is an issue with the recording setup.
 

volker

Well-known member
Joined
Mar 15, 2009
Messages
665
Location
Germany
Nice work! Did you compare your circuit to conventional cathode bias? As far as I know grid leak bias usually results in slightly higher noise, would be interesting to know by how much.
 

Heikki

Well-known member
Joined
Jul 31, 2008
Messages
387
Location
Finland
I can only conclude that either the usual ambient electromagnetic radiation is getting pickup by the transformer

You are correct. Shorting the transformer secondary gets rid of all the mains hum and it's harmonics.

That is a very good result for an ECC83 and an unshielded transformer. In my experience, hum and its harmonics does materially affect the noise figure so I think you could probably do 3dB better with a screened transformer.

For me 3 dB improvement isn't worth the 10 times higher price of a screened transformer.

Nice work! Did you compare your circuit to conventional cathode bias? As far as I know grid leak bias usually results in slightly higher noise, would be interesting to know by how much.

I actually have a almost identical version of this mic preamp where the only difference is that the tube stage is cathode biased. There is a marginal difference in noise but nothing significant. I'll make some measurements sometime this week and post them here. There is a difference in distortion compared between grid leak bias and cathode bias. THD is pretty much the same but grid leak bias has more 3rd harmonic. In my opinion the grid leak bias sounds better especially when recording drums but it probably doesn't have much to do with the distortion harmonic components.
 

mhelin

Well-known member
Joined
Mar 12, 2005
Messages
635
Location
Tampere, Finland
The Recom DC/DC converter for B+ is quite an expensive part (~50 EUR/USD), couldn't it be replaced with a some kind of CMOS inverter multiplier (if you parallel inverters you will get more current out of multiplier circuit)? If you are going to build several channels though then the price will be no issue... B+ 150V?
 

ruffrecords

Well-known member
GDIY Supporter
Joined
Nov 10, 2006
Messages
14,797
Location
Norfolk - UK
For me 3 dB improvement isn't worth the 10 times higher price of a screened transformer.
You can buy a screening can for less than the price of the transformer. I am sure you could fit the Neutrik into this one:

A262CAN | Transformer Screening Can | RS

In fact the combination is so attractive I might just try it as an option for the poor man's tube mixer.

Cheers

Ian
 

Heikki

Well-known member
Joined
Jul 31, 2008
Messages
387
Location
Finland
The Recom DC/DC converter for B+ is quite an expensive part (~50 EUR/USD), couldn't it be replaced with a some kind of CMOS inverter multiplier

Since B+ is a little bit over 200V and the plate current for each triode is about 0.5 mA, something like fig 11 here would probably work.

You can buy a screening can for less than the price of the transformer. I am sure you could fit the Neutrik into this one:

A262CAN | Transformer Screening Can | RS

Thanks for the link. If I ever build another one I might try out those screening cans.
 

Bo Deadly

Well-known member
Joined
Dec 22, 2015
Messages
3,078
Location
New Jersey, USA
Put the transformers in a steel box. Fine an electrical junction box. Even if it has holes for screws and such, unlike electric field radiation which can get through any gap, electromagnetic does not get through small holes as easily. Especially mains frequencies. Find two boxes where one fits within another for two layers.
 

Bo Deadly

Well-known member
Joined
Dec 22, 2015
Messages
3,078
Location
New Jersey, USA
Interesting concept....humbuccking input vformers. Never thought of that. How would you go about doing this? Stacked right angles?
Presumably they would just be wired using opposite wires. So if you have two transformers A and B each with primary wires 1 and 2 and signal source wires P and N, then you wire P to A1, A2 to B2, B2 to N. So at any moment the current is running one way through one transformer and the opposite way through the other. If the transformers are placed as close together as possible, any electromagnetic interference would induce currents in each transformer that are equal and opposite and therefore cancel each other out. Not sure how much the spacial orientation matters but I would guess orienting them in the same way would work best.
 

Heikki

Well-known member
Joined
Jul 31, 2008
Messages
387
Location
Finland

Latest posts

Top