HE-569 pre/eq 500 series

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pahstah

Well-known member
Joined
Sep 8, 2005
Messages
264
Location
Italy
Hi,

i've built a prototype for a helios "inspired" preamp for 500 series.
it works very well except for high gain settings (65-70 dB position) when hpf filter is engaged (it becomes an oscillator at hpf frequency), so i'm trying to find a workaround for that either removing the filter or changing the circuit.
the mic input is based on the 2128 circuit and the output is a 22113 amp in order to drive an output transformer, this gives me the capability to float the circuit at +-15v and to have a balanced output.
input transformer is Sowter 7490 and output transformer is a 2623 wired 1:2 or 1:1.
I'm also thinking about trying a mod to  the gain switch to change the gain resistors to both amps at the same time to see if i can have the same gain without going into distortion too early, it's not canon but i'll give it a try to see if it pleases me.
When everything is defined i will run a small batch of pcbs, so if someone is interested please leave a comment in this thread.
Also i f someone is interested in the amp pcbs only let me know.
here are some pictures:
IMG-0319.jpg


colored knobs

IMG-0311.jpg


IMG-0322.jpg


IMG-0331.jpg


2128 amp

IMG-0334.jpg


22113 amp

IMG-0333.jpg



Paolo


UPDATE 26/09/2020

Mouser Carts:

Main Board and Eq
https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=93d1f779d9

Gain resistors original
https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=88f53e2145
Gain resistors new
https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=1c562e767f
(choose one of the two)

He2128
https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=35ece9b98a

He22113
https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=c44ac54377

You will also need:
7490f Lustraphone M10 Input Transformer 1:10 with a 1:1 tap from SOWTER

Carnhill VTB9042 and VTB9043 inductors from http://www.audiomaintenance.com

EA2623 from Capigear.
https://capi-gear.com/catalog/product_info.php?cPath=21_37&products_id=33
or
TR2623 from avdaudio:
http://www.avdaudio.com/catalog.htm?id=1366

I have attached the main board schematic for reference
 

Attachments

  • He569ver3a.pdf
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pahstah

Well-known member
Joined
Sep 8, 2005
Messages
264
Location
Italy
update:

- changed the transistor in the hpf circuit: oscillation gone  ;D

- the original gain stage goes in distortion (not pleasant)  very early, so i found a workaround by changing the gain both at the first and the second stage simultaneously.
This gets rid of the distortion and give plenty of gain. I've tested this mod alongside my great river nv preamp and it is on par (with it with maybe a little less noise).
Now, the problem is how to implement it, the 500 series module have very little real estate on the frontplate so i have three options:

a)  order a dual concentric switch from grayhill, with has the pro of maximum flexibility for gain staging, cons very expensive and  requires a concentric knob

b) keep the original gain stage switch in the 1st amp and just add a +25 dB switch for the second stage: pro cheap and easy to implement, cons not very versatile and need to find a place on the already crowded frontplate for the switch.

c) put a 2 x 6  switch in place of the 1x12 gain switch to operate simultaneously both amp gainstages: pro easy to implement no extra space needed on the frontplate, cons only 6 switching positions 10dB each ( but so was the switch in the original unit),  can't operate the unit into distortion if needed.

thoghts?

Paolo
 

12afael

Well-known member
Joined
Aug 6, 2004
Messages
1,260
Location
Helsinki
I see some ribbon cables those run signal? Maybe you should use shielded cable. And maybe moving them could solve the problem.
 

pahstah

Well-known member
Joined
Sep 8, 2005
Messages
264
Location
Italy
12afael said:
I see some ribbon cables those run signal? Maybe you should use shielded cable. And maybe moving them could solve the problem.

yes, that is something to look into. i see shielded cable from gainswitch to amps in original helios 69 and i did the same in my double500 helios.

thank you
 

pahstah

Well-known member
Joined
Sep 8, 2005
Messages
264
Location
Italy
So...
what i've found is that the two units, my version and the helios 500 version, are nearly identical. small differences in the eq section and a little brighter sound in the original. They begin to distort at the same gain setting and the distortion is similar.
However, while the original version is stuck with the "historical" gain staging, i've provided my version with a switch that controls the gain in the second amplifier. that gives the unit the abiltiy to deliver undistorted 70 dB of gain, and, as  i've said before, is on pair with my great river neve clone.  I think that this solves the issue of "not enough gain" that affected the original units at the cost of a more  crowded frontplate.
 

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