pahstah

HE-569 pre/eq 500 series
« on: May 19, 2019, 06:31:18 AM »
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:


colored knobs







2128 amp



22113 amp




Paolo
« Last Edit: May 19, 2019, 06:38:22 AM by pahstah »


Vac11

Re: HE-569 pre/eq 500 series
« Reply #1 on: May 19, 2019, 10:15:44 AM »
Looks great, i am in. What is your personal opinion about the sound of the unit?

Thanks

M.
« Last Edit: May 19, 2019, 10:18:53 AM by Vac11 »

Rocinante

Re: HE-569 pre/eq 500 series
« Reply #2 on: May 19, 2019, 11:24:59 PM »
I have been waiting for you to bust out something like this?  I'd love a couple of your amp pcb's
If there's a harder way to do this, I haven't found it yet.

Winetree

Re: HE-569 pre/eq 500 series
« Reply #3 on: May 20, 2019, 12:52:24 AM »
I'd be in for a pair.

Re: HE-569 pre/eq 500 series
« Reply #4 on: May 20, 2019, 12:13:27 PM »
Nice!

pahstah

Re: HE-569 pre/eq 500 series
« Reply #5 on: May 22, 2019, 12:46:49 PM »
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

Re: HE-569 pre/eq 500 series
« Reply #6 on: May 22, 2019, 02:05:58 PM »
I see some ribbon cables those run signal? Maybe you should use shielded cable. And maybe moving them could solve the problem.
heavy metal is the law!!!

pahstah

Re: HE-569 pre/eq 500 series
« Reply #7 on: May 22, 2019, 03:57:39 PM »
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

12afael

Re: HE-569 pre/eq 500 series
« Reply #8 on: May 22, 2019, 04:37:50 PM »
Keep them short and far from transformers, coils.
heavy metal is the law!!!

okgb

Re: HE-569 pre/eq 500 series
« Reply #9 on: May 28, 2019, 09:32:39 AM »
I'd be in for a pair if there were face plates to go along with it , Looks great btw!
GKB Audio / Greg Boboski


pahstah

Re: HE-569 pre/eq 500 series
« Reply #10 on: May 30, 2019, 04:23:14 PM »
ready for a comparison  ;D


craigmorris74

Re: HE-569 pre/eq 500 series
« Reply #11 on: May 31, 2019, 09:32:10 AM »
I would be in for a pair.

OneRoomStudios

Re: HE-569 pre/eq 500 series
« Reply #12 on: May 31, 2019, 11:14:30 AM »
I'd be interested in a pair if it came with with a faceplate/metalwork.

pahstah

Re: HE-569 pre/eq 500 series
« Reply #13 on: June 04, 2019, 04:56:41 AM »
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.

duantro

Re: HE-569 pre/eq 500 series
« Reply #14 on: June 04, 2019, 04:26:06 PM »
Looks great! I'd be interested

MatthewF

Re: HE-569 pre/eq 500 series
« Reply #15 on: June 05, 2019, 06:25:42 PM »
This looks great! I'd be interested, especially if metalwork were included!

Best,

Matthew

Ricardus

Re: HE-569 pre/eq 500 series
« Reply #16 on: June 05, 2019, 10:21:21 PM »
Put me in for two.
Audio mastering for hire..

boji

Re: HE-569 pre/eq 500 series
« Reply #17 on: June 10, 2019, 09:27:46 PM »
I'd be in for a pair too!


Squeaky

Re: HE-569 pre/eq 500 series
« Reply #18 on: June 11, 2019, 03:25:30 AM »
Nice work, very efficient use of real estate. Clever idea having dual amp feedback switching.

jensenmann

Re: HE-569 pre/eq 500 series
« Reply #19 on: June 12, 2019, 04:51:49 PM »
Great stuff, Paolo. Congrats!

@ the others who think this is an ordering thread: how about asking decently before ordering?
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort


 

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