Akai AM-U1 modifications...

[gekkehenk]

Hi folks,

I got a nice cheap and for the money good sounding amplifier from Akai.

It's an Akai AM-U1

I know it's cheap but it peforms well!

My idea is to do some modifications to this amp for a cleaner, better sound and to improve stability and less distortion.
My idea's so far are:

- For stability i could change the block transformer for a toroidal model with slightly more amps.
- Changing the usual resisitors for metal film.
- I could change the M5218 opamp in the riaa circuit for a Burr Brown OPA2604AP opamp.
- Replace all elco's for new and better ones. Elna or eq.
- Change the filter circuit in the RIAA correction with Wima FKP2 capacitors
- Maybe some gold plated connections for the inputs and output
- Change the STK461 for a more modern one with less distortion.

But do you guys have any nice idea's to for this amp?


Manual for the STK461 STK461 pdf, STK461 description, STK461 datasheets, STK461 view ::: ALLDATASHEET :::
Manual for the M5218 M5218 pdf, M5218 description, M5218 datasheets, M5218 view ::: ALLDATASHEET :::


Would you know any nice modifications for this amp in the circuit to improve it?

Seems like a nice hobby challange project for this cheap amp!!

Below is the schematic from the pre and main amp. Below that we have the schematic for the RIAA circuit.

PRE and MAIN amp



RIAA




Thanx!

[jean-paul]

Re-use the case, spray it in the right color, fit good connectors and put in a nice TK2050 amp. AFAIK the transformer is a 2 x 22V type which is the right voltage for TK2050 amps when the secondary windings are paralleled. This is now really a mediocre amp at best. But if you are emotionally attached to it you could do the things you wrote. I doubt if you will be satisfied with the result though.

I recall these having pretty weak PCB material with tracks peeling loose when you unsoldered a component which was unusual at that time. I worked at Akai for some time, they did make some nice stuff but lost focus. Too bad.

[mjf]

hello.
i have an old sanyo dca 205 integrated amp here that uses the same stk 461 hybrid ic........did some mods.

quote:

- For stability i could change the block transformer for a toroidal model with slightly more amps.
.....not worth to try. stk 461 was a cheap opamp.

- Changing the usual resisitors for metal film.
..........yes if they are bad or out of tolerance............

- Replace all elco's for new and better ones. Elna or eq.
.......yes,that is what i did and mostly can recommend.


- Change the filter circuit in the RIAA correction with Wima FKP2 capacitors
........use caps with narrow tolerance,perhaps it helps.

- Maybe some gold plated connections for the inputs and output
.......luxury. looks good.

- Change the STK461 for a more modern one with less distortion.
.........i think it is easier to build a new chip amp, e.g. lm3886 or something else.

But do you guys have any nice idea's to for this amp?
......perhaps a dc-rf-filter unit in front of the primary.

[B]Would you know any nice modifications for this amp in the circuit to improve it?
............bigger caps in the signal path if correctly calculated. perhaps a bigger rectifier and bigger psu elcos.

greetings

[gekkehenk]

Thank you for your reply MJF. The bias is fixed, can it change without notice then?




This is what i come up with for the RIAA correction so far.

It's powered with +15 and -15 volts, so I remove the zener diodes and put a 78L15 and a 79L15 fixed regulator in place.

Then the riaa has a very stable power supply. I think that 100mA is more then enough for the chip and components but I will have to measure that.

Then replace the opamp with the burr brown OPA2604AP opamp.

Replace the polyester capacitors C104, C204, C105 and C205 in the filter circuit with WIMA FKP2 capacitors.

Replace input and output capacitors C102, C108, C202 and C208 with evox rifa 1uF polyester capacitors.

Replace the other caps in the RIAA with new ones from Elna.

And replace all resistors with metal film 1% resistors.

This way I think I can get the most out of the circuit without any major changes.


What do you think of this?

[sakis]

the major issue with this amplifier is the output stage .... the STK will always be an stk ...meaning that performance is average from most aspects ( average doesnt mean bad ....just average )

i would also check wiring and may be replace some... kill that speaker AB switch and go directly out ...then relocate zobel+inductor in the binding post ... return from there to 0 volt with a seperated cable see what else will take from wiring aspect to minimize crosstalks and EMI if any

so far i think that you are in the right track you may as well stop there .... the next step will be to loose the stk and go for a small discrete amp fited inside there ...

kind regards sakis

[mjf]

QUOTE
It's powered with +15 and -15 volts, so I remove the zener diodes and put a 78L15 and a 79L15 fixed regulator in place.
Then replace the opamp with the burr brown OPA2604AP opamp.
Replace the polyester capacitors C104, C204, C105 and C205 in the filter circuit with WIMA FKP2 capacitors.
Replace input and output capacitors C102, C108, C202 and C208 with evox rifa 1uF polyester capacitors.
Replace the other caps in the RIAA with new ones from Elna.
And replace all resistors with metal film 1% resistors.
This way I think I can get the most out of the circuit without any major changes.
What do you think of this? QUOTE

......i think you are a turntable fan.
if c101,c201 at the input are ceramic caps you can change them to mkt or mkp or polystyrol or......
perhaps you can use 0,47uF for input caps c102,c202.......only a suggestion.
and you can measure or simulate the riaa frequency response........

[gekkehenk]

......i think you are a turntable fan.
if c101,c201 at the input are ceramic caps you can change them to mkt or mkp or polystyrol or......
perhaps you can use 0,47uF for input caps c102,c202.......only a suggestion.
and you can measure or simulate the riaa frequency response........

yes, that true! I really want to get the most out of it. But not only the RIAA is going to be modified. I was thinking to use the same opamp for the pre-amplifier instead of the only one transistor in it.

The frequency response of the riaa correction without any modification is 30Hz-15Khz -1dB

C101 is a ceramic cap, C102 is an elco of 1uF.

My intention was to replace the input elco with a polyester cap from evox rifa beqause they have a RM of 5mm. Most others are a lot bigger. Maybe MKT is an option to, they also have a rm of 5mm.

The Ceramic cap can be replaced with a even better fkp2 polypropylene cap. They are also going to be fitted in the filter circuit.

[mjf]

QUOTE
My intention was to replace the input elco with a polyester cap

......yes,replace the in and output elcos with polyester(mkt) caps.......