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
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
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.
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.
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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?
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?
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.
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.
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.
Dont worry, I did catch itoooops, of course.
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.
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).
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).
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
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
Hi GeorgK.
I finally found the time to do the mod. And here are my findings:
The gain went up, no doubt about that. Using my active bass. In passive mode I can make it clip both visually and audible. In active mode I can just make it clip. So now the active gain is equal to the previous passive gain. Good job.
BUT I have come across some strange behaviour.
As I turn the master from 75 percent and all the way up, the gain indicator steady drops by 50 percent. The sound gets louder though.
???
I finally found the time to do the mod. And here are my findings:
The gain went up, no doubt about that. Using my active bass. In passive mode I can make it clip both visually and audible. In active mode I can just make it clip. So now the active gain is equal to the previous passive gain. Good job.
BUT I have come across some strange behaviour.
As I turn the master from 75 percent and all the way up, the gain indicator steady drops by 50 percent. The sound gets louder though.
???
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Im afraid not. I attached all I have earlier in the thread.
The LEDs are in the schematic, but not how its controlled.
Its like some kind of compressor, but LEDs only.
I can live with it, but it is very odd.
BTW I still think the gain is a tad low. I will try R24=5k.
Maybe I have a small trimpot somewhere...
The LEDs are in the schematic, but not how its controlled.
Its like some kind of compressor, but LEDs only.
I can live with it, but it is very odd.
BTW I still think the gain is a tad low. I will try R24=5k.
Maybe I have a small trimpot somewhere...
Hi dotneck335.
Do you think the audio improvement changing TL072 and caps is worht the hassle?
By coupling caps you mean; C21, C24, C8, C11, C29, C19, C30, C25, C1, C6, C3 and C27?
And by film you mean polypropylene, polyester or metal film. Just the cheapest I can get?
YES.OPA2134 vs TL072 --- -7db less noise (guitar source); 32 db less distortion; 50% faster.
Yes, those caps. Polypropylene where you can fit them (probably ≤ 2.2 uF) and polyester where you can't.
Are the opamps socketed PDIP-8s? If so, it should be an easy replacement. If they are soldered SOICs, I'd go even a step further and put in OPA1642s, which are even better than the 2134s.
Yes, those caps. Polypropylene where you can fit them (probably ≤ 2.2 uF) and polyester where you can't.
Are the opamps socketed PDIP-8s? If so, it should be an easy replacement. If they are soldered SOICs, I'd go even a step further and put in OPA1642s, which are even better than the 2134s.
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