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DIY MM card - no bass...

J

James Evans

Bedroom Bodger
Right then, acquired my obelisk 3 with no phono card in it, so thought I'd build one. Found the below schematic for a single supply opamp based mm stage:

MM Stage

The original power supply lines provided +30v so I've got a vbe dropping that to +20v and then lt1086 dropping it to +10v (thought I might try the AD8620 at some point which has max input of +13v)

Input and output caps, and small decoupling caps, are polypropylene, 10uf and 22uf caps are polyester, only lytic is the 100uf.

After a few startup niggles the damn thing works, huzzah, but, there's no bass, at all...

It's being used as a plugin card with this pre:

obelisk 3 pre

the only mods done to that pre are the input coupling caps have been swapped for 3.3uf polypropylene.

Gain on the MM card sounds about right - at 10 o'clock the bass-light signal is a nice listening level.

don't have a great deal of time to dedicate to swapping stuff around, so, any pointers as to where to start looking?

I'm thinking maybe the input and output caps should be larger than 3.3uf?

should the 22uf decoupling cap be bigger?

cheers in advance for any pointers

James
 
That's an odd design, where did if come from?

I think I'd prefer the 750R/0.1u to be buffered from the rest of the amp.

Anyway first check for component value errors or assembly errors in the first amps feedback loop and that passive network on the output.

Paul
 
No, but check the part you have in there really is c.180K, and not 17K8... which would kill the bass very effectively.
 
If this follows the same protocol as the old Nytech, the MC section is simply an add-on to the MM section.......
 
Hi Les, its the obelisk 3, which had removable cards for both MM and MC, neither of which I have or have been able to get hold of, hence the diy project...
 
Fundamentally RIAA requires bass boost and treble cut on top of overall amplification. In James' circuit the bass boost comes from the 178k/10k/22n network and the treble cut from the 750R/0u1 pairing on the output.

The 'no bass' problem applies to both channels, so it's unlikely to be an assembly error.

The likely possibilities are therefore,
  • a layout error.
  • a component value error in the bass equalisation.
  • a component value error in the treble equalisation.
  • a design problem, the suggested component values are wrong.

The latter seems unlikely since the Rod Elliot circuit is pretty similar, unless the opamp is unhappy in this application.

Anyway I suggest careful review of the circuit to ensure that reality follows schematic. Then lifting a leg of the 0u1 and listening, followed by the same with the 22n and reporting on how the sound changes.

Paul
 
Thanks Paul

One question actually - does component orientation matter in the RIAA circuit - I've got the 10kR and 22nf connected the opposite way around. I've also got the 4.7kR and 10uf decoupling components opposite to that in the schematic.

Another thing I noticed - the Rod Elliot circuit this is based on has a 100k resistor from output to grnd after the output coupling cap - this is missed on the schematic I used - does it need to be there?

Cheers

James
 
"Another thing I noticed - the Rod Elliot circuit this is based on has a 100k resistor from output to grnd after the output coupling cap - this is missed on the schematic I used - does it need to be there?"

That cap is there solely to ensure that there is no residual charge in the coupling cap when connecting the circuit to other items - a sort of dumping device.
 
aha, sorted it, it was of course my error... connected the 10uf cap at the wrong end of the feedback loop.

Bass is back in spades, along with a load of what I assume is mains interference... will have to look at the power/grounding arrangements at some point. Will give it a listen for the moment.

Cheers for all the help above
 
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