Aikido Vary Mu

[mik]

Hi gentelmen, some time ago,I have read here about a guy that has build a mic pre using the Aikido amplifier, so I have the idea to try to use the Fred Forsell Side Opto Side chain on this Amp.

http://img259.imageshack.us/img259/7580/schemaavmue2.jpg

Should have sense ?

my intention is tu use 6sl7 / 6sn7 tubes. and the total gain of the amp is arond 30 db with 1:1 output trafo

is this enough ?

thank you.

M.

 

[solder_city]

ive been looking at that schematic for ten minutes and i still cant figure out what it is supposed to do :?

its like parts of two different devices grafted together in a way that doesnt make sense to me.

are you compressing by changing the grid voltage on the 6SL7? its going to thump like crazy

 

[mik]

so in doesn't have sense .

M.

 

[Larrchild]

Single-ended varimu circuits thump like crazy.

 

[Guest]

[quote author="Larrchild"]Single-ended varimu circuits thump like crazy.[/quote]

If to control both stages it will thump less. But anyway like crazy. Also, it may oscillate by a control signal (kind of tremolo effect)

 

[mik]

ok, It will thump even if I'll put a diff. pair infront of the aikido amp ?

http://img267.imageshack.us/img267/8059/schemaavm2eg7.jpg

M.

 

[Larrchild]

With the diff-input, it will be smooth.
Good.

Add some fixed bias to that centertap also, though.

 

[mik]

are you mean at the trafo , or at the diff. pair ?

M.

 

[Larrchild]

input transformer CT can be used to add fixed bias as well as control voltage.

 

[mik]

I'll add a resistror between R1/R2 from 1K to 3K I guest, what do you think?

there is a whay to skip the interstage trafo ?

if not any advice for a good one ?

thank you

M.

 

[Larrchild]

Some of us think the interstage is good.
They sound different with just plate resistors.

You need to fix the bias on those grids somewhere in the flatter part of the tube's gain curve.
Experiment.

Here is some info on interstage use:
http://www.groupdiy.com/index.php?topic=24961&highlight=interstage

Also, you can put a milliamp meter across your R7 for gain reduction readings.

 

[mik]

THank you larrchild, very interesting discussion..

I'll probably use an LL1671 wired 2:1 as interstage, and an 6SN7GT as diff-pair.

all the best

M.

 

[Larrchild]

[quote author="mik"]ok, It will thump even if I'll put a diff. pair infront of the aikido amp ?

http://img267.imageshack.us/img267/8059/schemaavm2eg7.jpg

M.[/quote]
This is minor, but I just noticed that your input attenuator is a dead short to the outside world at minimum setting.=)

 

[mik]

yes you're right, I didn't compute it yet, but I'll use a switch, with a total loss of about 30db, it seams to me it should be adeguate.

if I'll use input iron to 1:1, interstage 2:1 output iron 2:1 I'll probably obtain a total of 30/32 db gain max

I'm making the PCBs right now, and I'm sure I have to tune some value here and there.

M.

 

[PRR]

As soon as you upset the bias on that lower tube, it is no longer "Aikido". You can build a better SE limiter much simpler than this.

And the gain won't change until tube is nearly cut-off. You are changing both Gm and load. What is the gain of this stage? Mu/2. How does Mu change with current? Very little. So gain is going to stay 30-25.... until 6SL7 Rp approaches 500K (R8||R9). That will happen around 0.03mA. You actually still have some voltage swing, but treble will go-away first due to stray capacitance.

Wire-up just the tubes and a pot to a -9V battery. I bet gain does not drop, but distortion increases.

This would make a lot more sense with a photoresistor, LA2a style.

 

[mik]

I've put a 6sn7 diff. pair in front of the amp coupled by a LL1671 inter trafo.

http://img267.imageshack.us/img267/8059/schemaavm2eg7.jpg

din't comput biasing yet , but I will follow your advice
things should be better, i guess

M.