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Modding preamp on Little Giant 1000
Modding preamp on Little Giant 1000
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Old 3rd January 2018, 06:54 PM   #1
Nordskov is offline Nordskov  Denmark
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Default Modding preamp on Little Giant 1000

Hi guys.

Amps are expensive in the small state of Denmark. So I have my eyes on a used Ashdown Little Giant 1000 for a small, portable practice amp.
Unfortunatly the preamp doesnt match the Digimod 1000 power amp inside.
According to many sources, the pre is kind of meek and cant feed the power stage properly.
Some german dude designed a buffer/gain to install in the effect loop. Thats kind of sloppy and I wonder if somthing can be done to the preamp instead.
I attached a schematic for the LG1000.
I will appreaciate it if someone could help me find the weak point if possible.
Maybe too little gain in one of the opamps or a voltage mismatch somewhere in the chain.

Another mod will enable bridge mode. There's a tutorial on PowerSofts site.

Thanks
Attached Files
File Type: pdf Ashdown Little Giant 350, 1000 Preamp Schematic.pdf (47.6 KB, 58 views)
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Old 3rd January 2018, 07:42 PM   #2
GeorgK is offline GeorgK  Austria
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Unfortunately, the schematic ends at the FX send, are there any gain or buffer stages afterwards? Somewhere the master volume must be connected. If it would be just a matter of level (but is it that simple?) you could easily make the output of the pre higher.
Voltage gain of the first stage around U3A is only about 3.
Possibly the designers felt that the FX loop should not be driven with too high levels?

edit: I think I found the mod you mentioned. Just a gain stage obviously. You can do this on the pre. I would even set pre-EQ gain higher, the EQ itself should have no problem handling the level.

Last edited by GeorgK; 3rd January 2018 at 08:05 PM.
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Old 3rd January 2018, 08:53 PM   #3
Nordskov is offline Nordskov  Denmark
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Hi GeorgK. You are fast
Thank you for looking into it.

You are right. I didnt notice the main volume was missing, as I was focused on the pre.
I attached the schematic of the back board. The actual main pot isnt drawn, but the gain stage between FXreturn and power amp is there.
The input impedance on the power amp is 10kOhm
Some sources claim that the power amp punch hard with another preamp connected to FX return. Im sure the pre is the culprit.
How much more gain do you think will do the trick? And how much more will the EQ accept?
What would be your approach and which components should be altered?
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File Type: pdf Ashdown Little Giant Back Board Schematic.pdf (56.4 KB, 30 views)
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Old 3rd January 2018, 09:18 PM   #4
GeorgK is offline GeorgK  Austria
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After the master volume there is a last stage with a voltage gain of 4,7 (2,2 for the LG350).
You could make this higher, this would not affect FX level. Simply make R80 (47k) larger. Try 8k2, this would result in a factor of 8,2.
If it just depends on the instrument and the amp is designed to work with basses of rather high output, it might make sense to do this directly at the input stage. I guess there is some level meter? If this is working independently from the master volume have a look at how much you get when turning up the gain control, this may help to make a decision.
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Old 4th January 2018, 02:32 AM   #5
PRR is offline PRR  United States
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Quote:
Originally Posted by GeorgK View Post
... make R80 (47k) larger. Try 8k2, this would result in a factor of 8,2.....
I think your decimal place slipped. 82K will make gain of 8.2.

Nordskov: In general, make R80 bigger in proportion to how much "MORE!" you think you need . Twice as much? 94K. 3X more? 141K. This does not have to be exact, you have a knob(s) for gain trim, round to nearest standard value. 100K, 150K, etc.
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Old 4th January 2018, 05:02 AM   #6
GeorgK is offline GeorgK  Austria
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Quote:
Originally Posted by PRR View Post
I think your decimal place slipped. 82K will make gain of 8.2.
oooops, of course.
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Old 4th January 2018, 08:09 AM   #7
Nordskov is offline Nordskov  Denmark
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Quote:
Originally Posted by GeorgK View Post
oooops, of course.
Dont worry, I did catch it
Thx PPR.

The amp isnt mine yet. I'm in the process of finding out what can be done to it before I throw money at it.

Others have commented that the led VU-indicator has a low reading. And you have to turn the gain trim all the way up to get it in clip mode.
The indicator reads the signal pre EQ (U4A - U4B) and post EQ (U5A - U5B).

Can I change R18 or R12 the same way, increasing gain early in the chain (U3A or U3B) as you suggested, GeorgK?

I'd rather fix the pre gain if its too low, instead of altering a circuit that works well.
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Old 4th January 2018, 12:51 PM   #8
GeorgK is offline GeorgK  Austria
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U3A is not that straight, although that would be my choice. Gain of this stage is determined by G=(R18+R24)/R24 - so currently 3.1
So to increase gain you can make R18 larger and/or R24 smaller. For example, with R24=1k8 you would get 6.5, or roughly 6dB more. If you need more, increase R18.

Changing R12 would work similar to the amp driver stage, but voltage gain of this stage is determined by more resistors (R25,26,12 and P1).
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Old 6th January 2018, 06:23 PM   #9
Nordskov is offline Nordskov  Denmark
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Now I have it home and I tried it with my active bass.
In passive mode I can just hit the red led, really hard plucking and gain turned all the way up.
In active mode I only get half way.
I guess there's about 10dB difference between the two modes (roughly double perceived volume to my ears)
So if I can increase input by roughly 6-10 dB I guess that will do the trick.
Will the input stage accept such an increase in gain or should I distribute it among the different stages?
Thank you for your help
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Old 6th January 2018, 07:49 PM   #10
GeorgK is offline GeorgK  Austria
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I see no reason why this should not work on the input stage alone.
You can try to solder 2k2 over R24, totalling in 1k5 , that would take the input stage to a gain of 7.6.
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