[gates_2]

Quote:

Originally Posted by amb /img/forum/go_quote.gif When you unplug the pot/attentuator from the ε22 backplane, the amp will have an open input except for 1M resistors (R2) going to ground. This is going to pick up noise, and is the hiss that you hear. When a pot is plugged in, at low volume positions the amp board's input is shunted to ground through a low resistance in the pot. Near the maximum position the input is shunted to the equivalent of "ground" (the low output impedance of the source). At middle position the input "sees" the paralleled value of the two pot sections to ground, and it also keeps the noise low. Now, with your stepped attenuator, the situation is similar, except that it's six cascaded stages of resistor dividers switched by the relays. I don't know what state each relay is in for any given position of the control, but if there is any circumstance that the amp board's input would "see" an open circuit or high impedance, then you'll hear noise.

Hmm... interesting...Perhaps this attenuator is just not a great fit for this amp(high impedance? hmm) I'm 95% sure there isn't an issue with assembly of the actual attenuator, like a bad solder join or something like that.

Quote:

Originally Posted by amb /img/forum/go_quote.gif With the attenuator set to minimum? Looking at its schematic, it would appear that there is no relay position that correspond to "fully off". In other words, its lowest volume position would still allows a little bit of signal through...

Yea, I know the attenuator doesn't have an "off" position. Actually, i'm hearing this with no attenuator plugged in at all...

 

[amb]

Quote:

Originally Posted by gates_2 /img/forum/go_quote.gif Hmm... interesting...Perhaps this attenuator is just not a great fit for this amp(high impedance? hmm) I'm 95% sure there isn't an issue with assembly of the actual attenuator, like a bad solder join or something like that.

FWIW, β22 isn't much different than most other amps as far as compatibility with stepped attenuators is concerned.

Quote:

Yea, I know the attenuator doesn't have an "off" position. Actually, i'm hearing this with no attenuator plugged in at all...

Again, if you don't have a pot or attenuator plugged in to the backplane, the amp board input traces on the backplane are essentially "floating", high impedance nodes, and could pick up crosstalk from adjacent traces. That is simply not a valid mode of operation.

 

[gates_2]

hmm... I guess I'm kinda stuck then...Perhaps its just the high(ish) gain with sensitive phones... Tried my K1000's and it sounds great

Thanks for your suggestions/help Ti

 

[MrMajestic2]

Sitting here staring at my massive component pile from JRossel, I have a few questions

Its a 4-channel kit with dual S22's by the way.

1. Im thinking about using a Joshua Tree attenuator for it. Do I need a separate transformer for this or is it ok to tap power from one of the S22's without adverse effect? It runs on lower voltage than the B22 so I will have to stick a TREAD or something in between of course.

2. When mounting speaker outputs do you guys use a switch between headphone and speaker outs?

3. When using speaker outputs do I or tap it before or after the E12 relay?

4. I want to have single ended inputs that convert to balanced. Whats the best way to do this? I have looked at some DRV134 boards but they seem hard to find. I think I can easily get my hands on Lundahl transformers since they are manufactured here, but I have no idea how to wire them.

 

[SoapSeller]

MrMajestic2, We on the same way.
I'm ordering parts from JRossel and have the Jouhua Tree on the way.

1. I've choose to use a separate trafo, But I don't know if it's really needed.

4. You can get DRV134 from assemblycraft, I'm going to build it with some THAT1646 that I've got from ebay

Cheers

Btw, For what do you need the E12 for?

 

[MrMajestic2]

Quote:

Originally Posted by SoapSeller /img/forum/go_quote.gif MrMajestic2, We on the same way. I'm ordering parts from JRossel and have the Jouhua Tree on the way. 1. I've choose to use a separate trafo, But I don't know if it's really needed. 4. You can get DRV134 from assemblycraft, I'm going to build it with some THAT1646 that I've got from ebay Cheers Btw, For what do you need the E12 for?

I e-mailed assemblycraft about the DRV134 some time ago but he doesnt reply.

The E12 is for headphone protection.

 

[amb]

Quote:

Originally Posted by MrMajestic2 /img/forum/go_quote.gif 1. Im thinking about using a Joshua Tree attenuator for it. Do I need a separate transformer for this or is it ok to tap power from one of the S22's without adverse effect? It runs on lower voltage than the B22 so I will have to stick a TREAD or something in between of course.

Looks like Joshua Tree has its own onboard rectifers and voltage regulator, and wants a 9V-15VAC single secondary transformer, not a DC power supply. Tapping off the σ22 won't be feasible. The Joshua Tree's current consumption is (200mA + (n * 150mA)) where n is the number of relay boards used (2 for balanced), so it's 0.5A in your case. Giving some headroom says you should use at least a 10VA transformer. As in any power transformer, you should place it well away from the β22 boards and the Joshua Tree relay boards to minimize magnetic field interference.

Quote:

2. When mounting speaker outputs do you guys use a switch between headphone and speaker outs?

If you want to be able to enable/disable either output then yes, otherwise you could always unplug headphones and speakers. If you use a switch, it should be heavy duty (I recommend at least 8A contact rating, preferably more).

Quote:

3. When using speaker outputs do I or tap it before or after the E12 relay?

Before. The small relay on ε12 is not rated for high enough current for speaker use.

Quote:

4. I want to have single ended inputs that convert to balanced. Whats the best way to do this? I have looked at some DRV134 boards but they seem hard to find.

It comes in DIP-8 package and requires very few external components, so you could either build one on a perfboard, or etch your own PCB for something a bit better. The circuit is simple enough that a single-sided board should suffice. You could tap off the σ22 and use 78L15 and 79L15 TO-92 voltage regulators to drop the voltage down to +/-15V locally on this board.

Quote:

I think I can easily get my hands on Lundahl transformers since they are manufactured here, but I have no idea how to wire them.

When you see the transformer it should be fairly obvious.

Both the DRV134 solution and the transformer solutions have their advantages and disadvantages. DRV134 will be more linear, have wider bandwidth, less phase shift, etc., but it's an active circuit and requires DC power, and may add a little bit to the noise floor. Transformer is passive, so no DC power necessary, but it's much more expensive, has intrinsic inductance which restricts bandwidth, reduces slew rate and introduce phase shifts. Depending on the particular model it may have magnetic nonlinearities.

Either way (DRV134 or transformer), you need to have a switch to select between balanced and unbalanced operation (in other words, switch the converter out of the balanced input path when you use balanced mode). This is so that your balanced source won't "see" the output of the converter which will have low impedance to ground.

 

[MrMajestic2]

Thanks for taking the time to write such an exhaustive reply.

Didnt realise that the JT needed AC input, guess Im back to the drawing board on that one. I could put another transformer in my S22 case, there is plenty of room

But at this point I already have two umbilicals so I think I have to redesign that part. Maybe an 8-pole Speakon, hmm.

Switching to speakers will then have to be done with big-a** relays then. Omron has the G2R at 16A, which should be sufficient.

Tapping before the E12, got it

Just checked the price for 2 Lundahl tranformers and I will now look harder for the DRV134 boards

Dang, those things are expensive.

 

[dbfreak]

"Roll Call: Who's building, built, or thinking of building a beta22?"

I am thinking (if I may), if they let me come up for air.

 

[koike]

ground channel has problems and the volume pot seems to pan left and right without attenuating any volume.

and i thought i could enjoy my weekend with my new hd600 and b22...

 

[MrMajestic2]

Quote:

Originally Posted by koike /img/forum/go_quote.gif ground channel has problems and the volume pot seems to pan left and right without attenuating any volume. and i thought i could enjoy my weekend with my new hd600 and b22...

What volume pot are you using? Are you sure its not a balance control?

 

[devin_mm]

Well I'm in the process of building my 6 channel b22 and I am thinking about volume control at the moment. Now to me the JT attenuator seems like a good way to go but I was wondering (for anyone with some experience with it) if I could have some sort of switch that combines two boards for balanced then when I want unbalanced it separates them into two independent controls?

 

[koike]

Quote:

Originally Posted by MrMajestic2 /img/forum/go_quote.gif What volume pot are you using? Are you sure its not a balance control?

its a 50KAX2 i ordered from glassjar...

 

[MrMajestic2]

Quote:

Originally Posted by koike /img/forum/go_quote.gif its a 50KAX2 i ordered from glassjar...

Is that the famous "fake" Alps pot? The behaviour does sound like its a balance control, but Im not sure how to check to make sure. Maybe it is broken.

 

[koike]

aw crap.
nah its a real alps pot.
might be broken.
i wired it correctly though.