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-   -   Tangent's ground channel w/o buffer (http://www.diyaudio.com/forums/solid-state/148102-tangents-ground-channel-w-o-buffer.html)

morfic 26th July 2009 09:43 PM

Tangent's ground channel w/o buffer
 
http://www.tangentsoft.net/elec/bitm...nd-channel.png

What would the schematic look like with regard to the opamp's feedback loop if i left out the buffer if i don't have current needs to need the buffer?

Do i pretty much do this: http://www.tangentsoft.net/elec/bitm...ds/vfb-opa.png

with this: http://www.tangentsoft.net/elec/bitm.../tle2426cp.png

And do i keep R3 at 1K instead of the 100Ohm if followed by the buffer?
Omitting Cc?

Thanks,

Daniel

morfic 26th July 2009 10:23 PM

R3 being 1kOhm and R1 being 100Ohm, either on the opamp's output.
I'm tempted to guess I'd be using 1kOhm, Cc i don't see hurting anything either way though?
Only because it was the value he used in all other circuits where the opamp was used to split the rails w/o being followed by the buffer.

jcx 26th July 2009 11:33 PM

check if the tle2426 can provide the current and low impedance you need in your circuit - I believe it can handle a few 10s of mA wthout additional op amp or buffer

op amp "gnd buffer" can be intersting to analyse for stability with capacitive load


there are also other supply spitting options - usally just using a dual supply or the midpoint of an (even numbered) battery stack is simpler

morfic 27th July 2009 01:16 AM

The TLE2426 pdf talks about a MAX condition of +-80mA, i think Tangent mentions 20-40mA.
I think that that would be enough and play nice with the LME49720's max current (I'll be testing the circuit with a OPA2134 first.
And from all i read the LME49720 will love some .1uF bypass caps (metal film?).
I like the idea of tangent's active ground/ground channel, but since he mentions to use the same opamp and buffer in the ground channel as is used for actual amp circuit, i figured to take out the buffer since i am not using a buffer for left/right either.

Rather than just creating a nicely split power source i was interested in the active output ground of this.
Just would want to skip the buffer in the schematic, unless it makes sense to run a LME49720 for L/R and the ground channel with tle+opamp+buffer, even if there isn't a buffer after the LME49720.

If my company had paid me Thursday i would likely have at least the parts and listen to the basic cmoy with the dip8 tle rail splitter off Tangent's side, but that's pretty woulda/coulda/shoulda ;)

So my question remains, can i strip his ground channel schematic of the buffer, or does it actually make sense to leave it even if the opamp for left and right channel does not have a buffer?

nelsonvandal 27th July 2009 07:51 AM

Is it a CMOY with active ground you're building, like a PINT or Mini3? When you say you don't have "current needs", what load are you thinking of?

LME49720 is peaking when run at unity gain. OPA2134 doesn't look too good either at unity gain. I'm not sure it will affect the sound.

I think it's a good idea to use different opamps in ground and L/R so you can blend the sound the way you want. Eg I find the combination of OPA134 and LME49720 to sound better than either of them used in both positions.

Sure you can use an opamp as active ground, but omit the 1k resistor in the feedback loop, or use a much lower value.

jcx 27th July 2009 04:20 PM

the TLE2426 data sheet is missing the critical "min output current" - the worst case (minimum) current it could sink/source so you have to derate the typical spec by "guesstimating" from the graphs - I wouldn't feel comfortable relying on more than 20mA from what I can see

the smallest short circuit spec is 31 mA - meaning the the output is only sinking that much while pulled all the way to the supply rail


echoing Nelson, you need to determine the active gnd load current requirement from the load impedance(s) and required output Vswing

morfic 27th July 2009 05:12 PM

Quote:

Originally posted by nelsonvandal
Is it a CMOY with active ground you're building, like a PINT or Mini3? When you say you don't have "current needs", what load are you thinking of?

LME49720 is peaking when run at unity gain. OPA2134 doesn't look too good either at unity gain. I'm not sure it will affect the sound.

I think it's a good idea to use different opamps in ground and L/R so you can blend the sound the way you want. Eg I find the combination of OPA134 and LME49720 to sound better than either of them used in both positions.

Sure you can use an opamp as active ground, but omit the 1k resistor in the feedback loop, or use a much lower value.

Right, it's a cmoy, with Tangent's active ground, basically just making sure i am not way off course omitting the buffer.
A friend built his and everything draws 10mA according to his MM, interestingly it pulls the same idle. So i hope i get similar results once done. The phones driven are both 32ohm.
I Will then either use the 100ohm used in the active ground schematic between opamp and buffer or none.
It;s going to be a growing project, a buffer might end up in there later, this question relates to me always feeling the need to ask question if i am at least close, as i get going i hope to pick up enough knowledge along the way to only ask when really stuck ;)
Your note about mixing opamps actually helps, this way i can look which of those i "know" (aka have listened to) are capable of more current for ground channel.

Quote:

Originally posted by jcx
the TLE2426 data sheet is missing the critical "min output current" - the worst case (minimum) current it could sink/source so you have to derate the typical spec by "guesstimating" from the graphs - I wouldn't feel comfortable relying on more than 20mA from what I can see

the smallest short circuit spec is 31 mA - meaning the the output is only sinking that much while pulled all the way to the supply rail


echoing Nelson, you need to determine the active gnd load current requirement from the load impedance(s) and required output Vswing


From what i understand the TLE does not have to deal with the driver's current needs, it deals with the small currents to vground only.
Omitting the buffer, instead of say 250mA i would be limited to whatever the current the opamp is capable of handling, LME49710 may not be the one i end up with in ground channel. It's not capable of much more than the TLE for example.
LME49720 just happens to be the most enjoyable opamp to listen to.

EDIT: I'm almost tempted to use LM6172/LM6171, it sounded real nice in a portable amp i had, but i got very irritable with it after a while, not until later that i read it's a temperamental little gem, I guess the circuit was not made to work with that fast an opamp.
Since Tangent does show how to work with them on another page, i may have to try those, once i get a slower one working nice ;)

Thanks to both of you,

Daniel

nelsonvandal 27th July 2009 06:57 PM

It's not always mentioned that the TLE2426 when used this way, sinking/sourcing the return currents, has it's own sound signature. I find it brighter than neutral and not as good as most of the modern opamps.

A cheap, fairly easy and good way (IMO the best way) to get the current you want is to use a discrete buffer in closed loop with the opamp. Since you use low impedance phones I think this is the way to go.

LM6171 has a warm sound signature and blends very well with the more "sterile" LM4562. I use this combination in a couple of amps with discrete buffers. The problem with LM6171 is the high input bias currents, but this is not an issue when used as ground channel. It's not as cranky as it's rumour. I would say the opposite, and it's cheap. Since it's got enough current for low impedance phones, you could get away without a buffer.

morfic 27th July 2009 08:37 PM

I Wasn't sure if the ground channel actually contributes to the overall sound or not, from what you are saying i gather it does, and thus i will do some mixing and matching.
I will certainly try the LM6171, it's added to my order already, added a LT1363 as well, since i also have a LT1364 i got with my hybrid amp, i have plenty of mix and match choices now.
Radioshack threw me a curveball of course, they no longer stock the pcb i wanted to use locally.
Since i will have to go mail order on the pcb, i am debating between the above and
this one.

Not sure if i need more "connections per pin" and could live with the single rail down the center. But since i need to go mail order may as well look around instead of jumping on the radioshack one (even if it IS the one on tangent's website)

Either way, i have enough extras in that order to build at least a second amp (if i forgo sockets on it), with the purpose of the extra parts being that i want to try different size input caps/power caps, different type bypass caps, various gains and once i have another power supply, different voltages.
Minimum rating is 35V with the psu i pick up later maxing out at 30V, so i guess it's all fun and games once i place the order ;)

morfic 28th July 2009 11:14 PM

Nelson, do you have some discrete buffers to suggest aside from sijosae's, impossible to google and not hit threads that in the end are based around his.
Got boards (went with the audioxpress ones) and parts finally ordered, no buffers yet. I hope i can pick up resistors locally and should have enough resistors and caps (if needed) left over from the amp part order.

Thanks,

Daniel


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