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
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
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
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