[skyskraper]

i found a gain of 7.5 to be just short of what i need. a gain of 11 should be fine

 

[DigiPete]

Quote:

Originally Posted by CPL593H What should I set the gain at on the M3 I'm building. I'll be using a home system CD source like my NAD502 or a Sony SACD player. The phones will be Sennheiser HD600s. Should I use the default gain of 11 or back off some? thanks

I found that the gain of 11 works well, because some SACDs are recorded at low levels, and I had to crank the volume pot up to about 70% (with my DT880s).

 

[Jaypetermen]

Quote:

Originally Posted by CPL593H What should I set the gain at on the M3 I'm building.

How do you adjust or set the gain to 11 on this amp?

 

[DigiPete]

Quote:

Originally Posted by Jaypetermen How do you adjust or set the gain to 11 on this amp?

from Amb's website on the M³:

Notes about R3 and R4

The voltage gain of the amplifier is determined by the ratio of R4L and R3L (as well as R4R and R3R). The default values of these are 10KΩ and 1KΩ, respectively, for a gain of 11. This is usable with most headphones, although it might be a little high for the most sensitive low-impedance types.

The gain may be reduced to suit such headphones. Further, the gain may also need changing depending on whether the amplifier is to be used with computer/portable devices or home stereo. The latter typically have a higher "line out", requiring less gain in the headphone amplifier.

The gain is computed by the formula:
G = (R4 / R3) + 1

 

[Jaypetermen]

Quote:

Originally Posted by DigiPete from Amb's website on the M³:

Thank you, found it on Amb's website under 'Parts List', 'Resistors', 'Notes about R3 and R4.'

 

[DigiPete]

Question:

I have been reading about the praises of the AD797ANZ and the Supermacro V3. Also been reading good things about the OPA604.

Will these op amps roll into the M³, at least from a functional standpoint, to allow comparisons?

Any arguments pro or against?

Thanks
DigiPete

 

[amb]

See my M³ site (under Parts list) for a list of tested opamps. The OPA604 is among them. I do not recommend using the AD797 here because it is not FET-input and has high input bias current. In the M³ there are no coupling capacitors, so the output DC offset voltage will be high, and will vary with the volume control setting.

 

[DigiPete]

Quote:

Originally Posted by amb See my M³ site (under Parts list) for a list of tested opamps. The OPA604 is among them. I do not recommend using the AD797 here because it is not FET-input and has high input bias current. In the M³ there are no coupling capacitors, so the output DC offset voltage will be high, and will vary with the volume control setting.

Amb,

Got it thanks. Not being an E.E. (only an M.E.) it helps to have opinions from people who actually understand the circuits.

Thanks much!
DigiPete

 

[steinchen]

opa604 ... hm ... power supply voltage up to +-25V

how does this opamp sound ? Can anybody give a short discription of its sonical character ?

where is the voltage limit of the M³ ? Caps 63V (electrolytic), tle2426 40V, opa604 50V, 2n3904 40V -> with a little safety margin of 2 or 3 Volts -> 38V max ?

 

[CPL593H]

Any suggestions for cutting the IEC AC power entry module holes in an aluminum box? I've got a drill press and roto zip. Both seem like crude tools for the task.

Has anyone used the Par Metal box and had Par machine the chassis holes? Was it affordable in the context of a $300 project?

 

[skyskraper]

draw outline of iec with pencil, drill inside the outline, then use something like a dremel to cut it to a proper square/rectangle. takes some time but the results are good, and its not too difficult.

 

[amb]

Quote:

Originally Posted by steinchen opa604 ... hm ... power supply voltage up to +-25V

Yup! If you want a lot of voltage swing this is the opamp to use in M³.

Quote:

how does this opamp sound ? Can anybody give a short discription of its sonical character ?

I'll let others do that, but during my short listen there wasn't anything glaringly bad about it. In fact it was quite good. This opamp is interesting in that all active amplification transistors inside are JFETs, and is the opamp used in most good NAD CD players.

Quote:

where is the voltage limit of the M³ ? Caps 63V (electrolytic), tle2426 40V, opa604 50V, 2n3904 40V -> with a little safety margin of 2 or 3 Volts -> 38V max ?

The 2N3904 bias transistor will never see the full rail, so that's of no concern. The MOSFETs are rated at 55V so it's ok, if you use 50V electrolytic caps then that's fine too. The TLE2426 is the "bottleneck" at 40V. Since the capacitance multiplier transistors and D1 cause a drop of another 2.5V or so, there is already a built-in safety margin and thus 40V is a good max PSU voltage. If you use a real split PSU and omit the TLE2426, then you could go further up to 48V if you have 50V rail capacitors.

 

[Ob3ron]

Hey i've got a few newbie questions regarding the wiring/grounding...

a) I'm going with AMB's wiring scheme 1 and am wondering how I might go about connecting the pot to my parmetals chassis if i've placed the board too far back to actually screw the pot shaft's nut onto the front panel. Would connecting the pot's ground screw to the input ground suffice since input ground will be connected to the case via the input jacks?

b) For a power switch, I've got an SPDT that says its rated for 6A 125V (I'm running a headphone MMM so that should be fine). Can I just put it inline with the AC live wire (one side going to the middle prong, the other going to the top or bottom)?

c) I'm making a dual input MMM and I've got a DPDT switch for input selection. I read in some thread (can't seem to find it now), that in theory it's not the best idea to have my two sources' grounds connected as are by using a DPDT, but practically, I shouldn't have any problems. Is this correct?

Thanks

 

[amb]

Quote:

Originally Posted by Ob3ron Would connecting the pot's ground screw to the input ground suffice since input ground will be connected to the case via the input jacks?

Yes. That's fine.

Quote:

Can I just put it inline with the AC live wire (one side going to the middle prong, the other going to the top or bottom)?

If by "prong" you mean the solder tabs of the switch, then yes. Use heat-shrink to insulate the wires afterwards so that no high voltage is exposed.

Quote:

c) I'm making a dual input MMM and I've got a DPDT switch for input selection. I read in some thread (can't seem to find it now), that in theory it's not the best idea to have my two sources' grounds connected as are by using a DPDT, but practically, I shouldn't have any problems. Is this correct?

There is no problem connecting the grounds of the two sources together and only switch the signals. That's how most preamps do it.

 

[Jaypetermen]

I am about to start building a M3 & STEPs. I currently have the AD8610AR op-amps (which are rated up to 27VDC) and I am also thinking about building a Millit-Hybrid, since I could not resist the group buy.

I would like to configure the STEPs to be able to power both amps (one at a time of course) without changing the voltage. Would 27VDC be a good setpoint for both of the stock amps?