Some G9 questions

[gyraf]

thanks a lot gustav

Hi Maxime,

Actually I'm Jakob - the one that draws the PCB's - Gustav is the one actually manufacturing them.. :grin:

Jakob E.

 

[Neeno]

[quote author="gyraf"]Maxime,

Newark-through-Farnell has the original Thomas&Betts Augat/Alcoswitch PKES-type knobs. Also Musikding.de seems to have some good ones..

Neeno,

It's good enough. Don't worry about it.

Jakob E.[/quote]

Hi there Jakob !
I know it's good enough... it sounds beautiful as a tube mic preamp !
You've done a great job, as always and as every your project...

Now i'm trying to use it to warm-up a little my masters...
Actually i don't have any tube equalizer or compressor i can use on my masters so i've tried to use the G9 as a makeup stage after a pair 1176.

What i've noticed is a smooth roll-of on low frequencies, so i've tried to do some measurement using RMAA (i know that it's not a professional measurement equipment but...) the result of the freq responce is a almost linear preamp with a very smooth roll-off under 300Hz...

I know that this may not be the right way to use the G9... and that's the reason why i'm asking...

Is this due to an impedence problem (i have the same results also using the hi-z input) or is this design related ?

Thanks for your help !

Neeeeeeeeeeeno :grin:

 

[maxime]

sorry Jakob
have a good night
maxime

 

[b2]

Hi,
I've some Dual unit G9 on Gyraf Base and I finish a Quad.

My G9 Base used just 6 positions on rotary Gain switche (1 positionR=12K to 6 positionsR=680) for my use it was more easy.
I've measured the Gain of each step and it's around 4 /4.5 dBu of amplification.

I do not used the PSU transformers 220v/15v and 15v/220v.
I've got a special hand made one by Magnetics in France wich work very fine.
My transformers are OEP A2 and A3 the A2 is reversed !!!!!
I've done a ZOBEL network on the secondary of the A3 at the input.with it I've measured Frequency response an it begin at 30Hz ,-4dBu go to 100Hz ,0dbu to 1Khz Linear to 16 Khz a boost + 2dBu at 22Khz and go down after 25Khz.

The Zobel network is R1 in serie With C in serie with R2
The value are R1=R2=22K and C=680pf Styroflex one!!

The Gain Max of this preamp is 56dBu.
I've done my unit with 12au7 and 12at7 all works very very fine.
On the PSU unit I've put caps 47nf/63v on all the rectifier Bridge to reduced noise,and to supply all heat in 12v I used the 78T12 regulator ,it can supply 3A on 12V with a beautiful Heatshink.
I'will thanks Gyraf Audio for all!!!and Many Many thanks for the Prodigy Forum!!!!
The G9 preamp is an EXCELLENT PREAMP.

Merci.
b2.

 

[maxime]

hi everyone
I am at the ordering moment and I was wondering about the HT regulator. RS Components has only the TL783CKC, which from the datasheets only outputs 115Volts max it seems, not 245.
I am sure I am missing something...
if someone can wipe my doubts I 'd glad.
have a good day you folks
Maxime

 

[gyraf]

Hi Maxime,

Read the G9 page:

The 245v HT is made from 220V/50mA ac, rectified and regulated by IC1, a TL783 high voltage regulator. The TL783 is limited to max. 120V in-to-out differential voltage, so the three 39V zener diodes (D3-5) protect it from voltages higher than this. Also the 1N4004 (D2) protects it from lower voltage on the input than on the output, a condition it hates.

So we use the TL783 in floating configuration, with over-voltage protection. It works excellent.

Jakob E.

 

[maxime]

super
great
many thanks jakob
good week end !

 

[steve]

My G9 is finished and functioning -- thank you Jakob for a well designed and documented project.

Here are some pics:

top view
http://www.photo.net/photodb/photo?photo_id=3538827
front view
http://www.photo.net/photodb/photo?photo_id=3538803

I thought I should share my experience with two of the more common G9 problems: 1) the oscillation problem at the highest gain positions; and 2) the low frequency hum problem.

THE OSCILLATION PROBLEM
When I first got the G9 working, the oscillation occurred in channel 2 at the two highest gain positions: at gain level 10 but only with the output control hard clockwise, and at gain level 11 at all points of the output control. Channel 1 was not as bad as channel 2, with the problem only occurring at gain level 12. The problem was worst at the 48v position, less so at the Mic position, and non existent at the Line position (with a guitar plugged into the front line jack).

During this first test, the G9 was connected (single ended) downstream to a Lexicon MPX110 sound processor (for monitoring through its headphone jack), and I could cause the oscillation to start by slightly increasing either the output control of the G9 or the input pot of the Lexicon.

What does the oscillation sound like? Well, first of all, no input signal comes through -- it just seems to be blocked out. What does come through is a whistling sound, sort of like tuning a shortwave radio, with very slight changes in the output level causing the whistles to shift. The amplitude stays the same, unaffected by the position of the gain level or output level.

This problem appears to be solved by reducing the load on the output. This is based on subsequent testing with other downstream devices as shown below:

- single ended output to Lexicon MPX110 (500K input impedance) -- oscillation occurs as described above
- single ended output to Peavey Bandit guitar amp (250K input) -- no oscillation!
- single ended output to Peavey Bandit guitar amp (44K input) - no oscillation!
- single ended output to Peavey Bandit guitar amp (22K input) - low hum problem, but no oscillation

I have not tested the G9 with a balanced downstream load.

THE HUM PROBLEM
The problem here was a heavy hum coming through the right channel. Touching the (insulated!) output wires or control board wires causes the hum to increase.

The solution to the hum problem included the following remedies:
- lowering (against the board/chassis as far a possible) the routing of the leads from output connectors K3/K103 to the output XLRs
- lowering (toward the bottom of the chassis as far as possible) the 10 pin and 5 pin leads going from the main board to the control boards
- changing the power supply caps (C14, C15) from 100uF to 200uF (as was suggested in a previous G9 post)

On my unit I have all the power supply leads running up over the board then down to their points of connection, and all the signal wires running down low as close to the board as possible. When I arch the signal wires over the top of the board I definitely get excessive hum.

I hope the above helps future builders of this fine unit. The beautiful nuances in guitar tone that my G9 achieves is remarkable.

Steve L.

 

[gyraf]

Hi Steve,

That is a good post indeed - I'm sure it will help future builders cope with common problems!

:thumb:

Thanks!

Jakob E.

 

[StrayCat]

These were my EXACT symptoms for my G9. I'll give this a try when I get back from vacation. I still did not have to use the top two detents so I did not see it as a problem. I have plenty of gain for my mics and DI purposes. I do have a slight microphonic if I tap the chassis at high gain or tap the tubes(with the eraser end of a pencil) but these are used tubes for testing purposes.

Cheers

 

[maxime]

hello Jakob and others,
I am at the end of pcb soldering on my 2 G9. great!
I was wondering how the preamp and its lundhal transformers behave with difficult loads such as ribbon mics and very low impedance (schoeps) mics.
I hope the summer is great to you all
bye
m