Blackstar Artisan Negative Feedback Loop

bennyg85

Hi

Has anyone removed the negative feedback loop from their Artisan 15?

If so, how is the NFB loop circuit arranged in this amp?

Many thanks

Ben

ecc83

I feel it would not be giving anything away to say, what you propose cannot be done the to  A15 or A30. You could do it to the A100!

Dave.

knuckleberryfinn

Are you sure it has a negative feedback loop? Should be very easy to spot - look at the speaker jack terminals, one will have a wire from the OT and also another wire going to a resistor. But I thought these were AC15-a-likes, which I'm pretty sure don't have negative feedback.

knuckleberryfinn

@Ecc83 will know. Are you there Dave...?

Misterg

knuckleberryfinn said:

@Ecc83 will know. Are you there Dave...?

He's BEHIND YOU!!

bennyg85

Thanks for the replies.

Why is it not possible with the 15 or 30?

bennyg85

Blackstar have confirmed it has one!

Misterg

There are a number of reverse-engineered schematics around for the 15 & 30 which don't show negative feedback from output to PI, which I guess is what you're expecting. They do show local negative feedback around the EF86(?):

Take them with a huge pinch of salt...

(posted on MEF - Note obvious error on EL84 cathodes).

http://img573.imageshack.us/img573/9825/artisan15part1.gif

http://img577.imageshack.us/img577/5968/artisan15part2.gif

(Posted on EL34world - drawn in a confusing manner!)

http://i255.photobucket.com/albums/hh143/werdnayarg/webstuf/Blackstar-Artisan-15H_zps115986b9.gif

I believe that Dave (ECC83) worked (works?) for Blackstar...

ecc83

Misterg said:

There are a number of reverse-engineered schematics around for the 15 & 30 which don't show negative feedback from output to PI, which I guess is what you're expecting. They do show local negative feedback around the EF86(?):

Take them with a huge pinch of salt...

(posted on MEF - Note obvious error on EL84 cathodes).

http://img573.imageshack.us/img573/9825/artisan15part1.gif

http://img577.imageshack.us/img577/5968/artisan15part2.gif

(Posted on EL34world - drawn in a confusing manner!)

http://i255.photobucket.com/albums/hh143/werdnayarg/webstuf/Blackstar-Artisan-15H_zps115986b9.gif

I believe that Dave (ECC83) worked (works?) for Blackstar...

I did indeed. Twice! I had to jack in Sept 13 due to a serious family health issue.

I do not think the Artisan 15 and 30s are a deliberate attempt to "clone" a Vox? They just use a similar valve lineup (tho' I think the EF86 stage is quite unique?) .

We have a Mersey Super 15 which is itself a Dominator clone but Son says it does not "do" AC15 and it CERTAINLY does not sound like an Artisan 15! He loved the EF86 stage in the Blackstar amp.

The Artisan 100 is different again and is really only a very well build "classic" EL34 amplifier. They seem to me to have been rather ignored and therefore might be good value on the Bay? Certainly if you can find a good set of valves for one you will never have any other trouble!

Dave.

bennyg85

Hi Dave

Thanks for the reply.

Can you explain your original comment about removing the NFB loop please?

Thanks

Ben

jpfamps

bennyg85 said:

Hi Dave

Thanks for the reply.

Can you explain your original comment about removing the NFB loop please?

Thanks

Ben

There is no NFB to remove!

bennyg85

That's not what Blackstar say.

jpfamps

The schematics I've found on the web don't show a feedback connection from the speaker outs.

bennyg85

Those schematics are generally considered to be wrong.

I'll get a tech/expert to have a look and come back with my findings.

Ben

jpfamps

ECC83 used to work at Blackstar, and he says it can't be done.

Although he is coy about the reasons (pointlessly in my view), the only logical reason I can think of for not being able to disconnect the NFB loop is because there isn't one.

Misterg

There isn't Bassman type negative feedback in those schematics, but there *is* a negative feedback loop (so Blackstar aren't lying). It's around the EF86 and I'm pretty sure you don't want to disable it, because you will end up with a shit load (and I mean a shit load) more gain - probably turn the amp into a 15W fuzz box.

You can't disable it by removing components ('lifting' them or cutting their leads) - you would need to find and break the connection marked 'X' below:

http://i255.photobucket.com/albums/hh143/werdnayarg/webstuf/artisan15part1x_zpsb2a3fa89.jpg

What are you hoping to gain by this?

Looking forward to your expert's input.

ecc83

jpfamps said:

ECC83 used to work at Blackstar, and he says it can't be done.

Although he is coy about the reasons (pointlessly in my view), the only logical reason I can think of for not being able to disconnect the NFB loop is because there isn't one.

Coy? Maybe, I call it old fashioned loyalty. When/if Blackstar decide to publish their schematics I shall be happy to discuss them. I am happy to give certain information, bias values for instance. I consider the OP's idea "pointless" and I know Blackstar are not keen on people modding their designs.

My own PERSONAL view is that manufacturers of all persuasions should make service information readily available (Quad always included a schematic in the user handbook. Din't do them any harm!) but I respect Blackstar's position.

Dave.

bennyg85

Why is it pointless? I'm told there is NFB and I'd like to add a switch to remove it.

ICBM

Misterg said:

You can't disable it by removing components ('lifting' them or cutting their leads) - you would need to find and break the connection marked 'X' below:

A better way to do it would be to bypass (shunt) the 470K resistor between the NFB connection and ground (while keeping the 470K in series with the loop, or it will kill the signal). Best of all, make it variable - ie connect the 1M resistor to the rotor of a 470K pot - that way you could control the amount of NFB.

I don't know exactly what that would do to the sound or necessarily be desirable though - other than definitely giving a lot more gain. So I would try to hook it up temporarily and not do anything destructive until you find out. Don't mess up such a well-made amp if you don't have to.

bennyg85

Thanks for the helpful reply.

My tech has the schematic now; he's just trying to find time to look at it.

I'll keep you posted!

Ben

martinw

Although the local NFB loop shown around the EF86 is indeed an 'NFB loop', it's not the Global NFB loop that most people would be referring to in the context of making NFB switchable.

Local NFB like this is not uncommon, but as has been mentioned, the idea of simply switching it off doesn't stack up, as the effects are likely to be undesirable.

Unlike a global NFB, where making it defeatable or variable can be an interesting way to add tonal flavours to an amp.