EL84 Mic Pre

[ruffrecords]

Sorry Ian my error, it was -63dB at max gain, not -83dB and so E.I.N was -114dB about the same as a V72.
It was about -83dB at the gain I use for my condensor mic.  Sorry for the confusion.

best
Dave

No problem Dave. -114dBu is still a respectable figure. I use Sowter's 10:1 transformer at the input to my designs which has another 8dB of almost noiseless gain and I get about -122dBu which seems to stack up with your measurements.

Cheers

Ian

 

[gtrecords]

Hi Dave... Nice Project!!! can´t see the schematics , can you upload again ?

 

[Tubetec]

Ive made a pre with a single triode connected EL84 and Sowter 9705 , took a large amount of Henries and uF's to banish hum from the plate supply  of 205 volts ,Dc heaters as well ,runs with a little over 30ma of plate current . I went with  1.1 megohm on the grid,a good bit above what the data sheet recommends ,no problems with that though. For mic input I use a stepup Tx at the input followed by an inline socket/plug with the appropriate matching resistance in parallel with the grid resistor. It also works great as a Hi into Lo Di type unit for passive pickups or if more gain is required easy to patch in another gain stage at the front end . Direct to the grid bandwidth approaches 100khz and the overload characteristics are smooth as silk .

 

[DaveP]

Hi Dave... Nice Project!!! can´t see the schematics , can you upload again ?

This was the first version:-

Later on I needed the ECC88 for another project so I subbed a triode wired EF86:-

I put in the Input impedance switch after reading about how SM57's had a better sound loaded with 500 ohms for drums.
Soon after that I gave up recording drums for a drum machine!  It is probably better to have no termination resistance on the IPT, like they did in the old days.  Less noise too.

You can get a 25VA toroidal Power TX from Farnell nowadays so no need for the double set-up I used anymore.
http://uk.farnell.com/triad-magnetics/vpt230-110/transformer

Also with Ultra fast diodes you don't need the caps on the rectifiers either.

DaveP

 

[Tubetec]

Looks like a nice setup Dave.
I like being able to vary the matching on my gear as well ,provides a subtle but usable form of tone/colour control by loading/unloading  the transformer core.

 

[lassoharp]

It's the pleasant overdrive characteristics of the EL84 that gives this circuit its signature. 

I've built numerous variations including some using pentode mode. 

Noise is a little higher but I've heard noisier modern SS mic pres

Great design Dave!

 

[Alm6mountain]

Been excited by this schematic for a while now and finally got around to building one with the EF806 as the input tube (which I expected to be similar in performance to the 86 as it appears on the data shee). I left out the leak resistor off of the input transformer and have found amazing results from every mic I plugged in. I have been attempting to add a DI for a few days now and have noticed quite a bit of hum. The mic signal following the schem is more or less hum free. I have tried multiple ways of grounding the DI to reduce hum and have come up empty handed. I even tried the cinemag DBX DI transformer on the input and it seemed to yield the same result of hum. I found a schematic for a guitar amp input that I think may be a solution. Lowering the VDC at pin 9 by adding a 470k resistor and a .047 electrolytic to ground. I am not sure what the effect will be, but I would love to hear any other ideas that would make this DI in sound as good as the mic input side. I will be sure to update this thread on my findings.

here is a link to the schem that lowers the VDC at pin 9.
https://www.ampbooks.com/mobile/amp-technology/EF86-power-amp/EF86-power-amp-bench-test.gif