v76:isch preamp

[solkatten]

Hi there again. I got an itch to use my stored Telefunken ef804s tubes in a mic pre. I will re-use chassi and supply from my planar v72 pre: The planar v72 from scratch

Im building it with parts at hand, so some mods is unavoidable .

The idea is to use the ef804s as the input tube and use the classic and probably the best feedback arrangement from the v76 design. The second tube is changed to half a 6n6p, wich use a resistor instead of a the original anode choke.

In between input and output stage is HP och LP filter. I dont need a shitload of gain so the last stage is cathode follower 6n6p + LL1671se. It drives the transformer pretty much flat into 30-40Khz region.

The goal is stability, low THD and great bandwith.

I will also use my handwound 1:8 IPT: The Mad Science Challange -DIY MIC IPT

 

[ruffrecords]

75R and 100K implies a desired closed loop gain of more than 60dB. I would be surprised if you get that much more than that open loop. Don't forget you have 18dB gain in the 1:8 transformer.

Cheers

Ian

 

[gyraf]

Interesting idea. Respect from here for IRL'ing..

I'd shunt the output stage HP/LP filter with a pot, making it gradually variable - more usable that way

 

[solkatten]

Interesting idea. Respect from here for IRL'ing..

I'd shunt the output stage HP/LP filter with a pot, making it gradually variable - more usable that way

Great idea... How do things go with toroid widing machine?

 

[solkatten]

75R and 100K implies a desired closed loop gain of more than 60dB. I would be surprised if you get that much more than that open loop. Don't forget you have 18dB gain in the 1:8 transformer.

Cheers

Ian

Hi I have to recalculated the feedback resistors, gain should be in the 50dB range

 

[solkatten]

I think this design should mature into a channel strip. V76 inspired pre with simple eq. LP, HP hi and lo boost. The LL 1671 acts as an interstage 1:1 into a fairchild inspired compressor like kingstons m670.

The chassi is drilled for 1 ch, but it has a big PSU and space left

I got a UTM 3580 PP for compressor OPT and 2 edcor 1:5 for side chain amp

 

[solkatten]

I think I will make a diy transfomer for double duty and save one trafo. It will have dual secondarys it will be 15K:600 for compressor output, but also 15K:3K75 for the control amplifier. 5:1+2,5 trafo. I will use the same vitroperm 500F core as my IPT.


Primary:
Singel layer 0,05mm 275H
Electrastatic screen
1. Secondary 0,25mm 11H
2. Secondary sectioned 0,1mm 69H

Another option UTM2660 interstage trafo. 4,5:1+2

 

[solkatten]

A cheaper output stage. Parafeed UTM 3580.

 

[solkatten]

Updated version of my channel strip. I borrowed a tilting eq design from Kingstons http://www.michaelkingston.fi/files/Drive-1_schematic_rev2.1.pdf. After the preamp/eq im gonna incorporate m670 compressor: M670 compressor credits to Kingston. The only mod I will do is dc-coupled the controll amp for max voltage swing, because my psu is max 230vdc. Instead of 5687 i will use 6n6p. 190Vdc regulated for compressor amp.

Transformers:
1:8 DIY IPT
2:1 LL1671SE as interstage pre out/ comp in
DIY 5:1+2,5 Audio output/ controll amp in
4:1 VTB-2291 controll amp out

Tubes:
EF804 x 1
12ax7 x 1
6n6p x 3
6n5p x 2

PSU
230vdc a 200ma
6,3vac a 4A
-12vdc for compressor

 

[solkatten]

I run out of space, so my active eq plan is scraped. Heres a passive EQ I have used in other preamps to "tilt" the sound brighter or darker. I wind my own inductors so 50H value is not a problem... If you wind toroid cores in full layer to reach the specific value they are very immune to noise pickup. With 100k to ground the currents trough the inductor is very small so distortion in minimal.

 

[gyraf]

again, if you short these with a (e.g. 1M neg.log.) pot, you can make them variable - providing, off course that you have the front panel space for knobs

 

[solkatten]

again, if you short these with a (e.g. 1M neg.log.) pot, you can make them variable - providing, off course that you have the front panel space for knobs

I think in this setup you should also scale the 47K resistor for consistent boost?
82k +47k, 150K+100K....

 

[solkatten]

Maybe like this. Switching both resistens with DPDT

 

[solkatten]

I think I have a plan. 3 knobs solution. Hi cut/off/boost LO cut/off/boost Z hi/lo

 

[solkatten]

Preamp card

 

[solkatten]

Hook up scheme for the eq section. Its a mixed filter: boost is a shelf and cut is a passfilter. As Jacob suggested I implemented a impedance switch to shift the selected freq.

 

[solkatten]

The preamp is up and running besides a faulty gain switch, wich I have to re-do. When that is sortet out I will do some measurments and see if the pre is stable under max gain reduction and THD and freq response plot. And I will assemble the eq section...

 

[solkatten]

Gain switch is now ok. The EQ is very gentle. And it is a little bit crouded in the front.

 

[solkatten]

The preamp measured in REW. Ca. 0db. Max gain reduction, nothing funky is happening... THD 0,03%. Flat freq response…