[Clutz]

Greetings all,

This will likely demonstrate yet again my poor understanding of electronics.

One of the standard reasons given why a Cmoy headphone amplifier does not amplify low Z cans as effectively as high Z cans is because low Z cans need lots of current and the Cmoy headphone amp increases voltage gain. Hence, if people want to power low Z cans like Grados the use of a buffer is often advised in order to provide the necessary/desired current.

Why not take a high voltage gain circuit - maybe using something like a Cmoy with 637s in it and a pretty high voltage gain, and connect it to an output transformer to decrease the voltage and increase the current. I realize there must be a good reason for doing this, but it is unclear to me what that is. For a while I thought that this must be due to the fact that transformers are adding non-linearities into the audio signal and that's a big no-no, but I recently read a post (I believe it was by Garbz) where he made a device to convert a balenced to unbalanced source (or vice versa) using transformer - which suggests that there must be another reason for not using transformers.

Thanks so much,
Cheers,
Clutz

 

[dsavitsk]

Quote:

Originally Posted by Clutz Why not take a high voltage gain circuit - maybe using something like a Cmoy with 637s in it and a pretty high voltage gain, and connect it to an output transformer to decrease the voltage and increase the current.

Good quality output transformers are very expensive, many times the cost of a cmoy.

 

[Clutz]

Quote:

Originally Posted by dsavitsk Good quality output transformers are very expensive, many times the cost of a cmoy.

How expensive is expensive? Is expense the only real factor? So expensive that it wouldn't make sense to match it with a Cmoy level voltage gain stage? Are there alternate voltage gain stages I could try instead - solid state, not tubed

Thanks for yhour help!

Cheers,
Clutz

 

[rickcr42]

Quote:

One of the standard reasons given why a Cmoy headphone amplifier does not amplify low Z cans as effectively as high Z cans is because low Z cans need lots of current and the Cmoy headphone amp increases voltage gain. Hence, if people want to power low Z cans like Grados the use of a buffer is often advised in order to provide the necessary/desired current.

the premise is a correct one but you are attacking it all wrong !

Just use a high current output opamp like the Analog Devices AD8397 as a single stage headphone amp with the gain set at no more than +6dB.
Not only are output transformers expensive but those with a 32 ohm output VERY expensive and to add them to a simple opamp just to drive headphones not a good cost vs. performance ratio.

 

[dsavitsk]

Quote:

Originally Posted by rickcr42 Not only are output transformers expensive but those with a 32 ohm output VERY expensive and to add them to a simple opamp just to drive headphones not a good cost vs. performance ratio.

Rick,

As a side question, do you know of any particular transformers with 32 ohm outputs that you would recommend? I am working on a preamp based on Pete Millett's low mu design using the 6080 tube. According to Pete, as I understand it, the output impedence of the preamp should be about 280 ohms. This is plenty low for a preamp, but I thought that perhaps hooking up an external transformer would make for a decent way to drive some grado's.

 

[Garbz]

weeeee doubel post big guy, you're a bit trigger happy

By expensive since no one's said it means it cost about $150USD for the transformers. And these are in one of the cheapest implimentations that I've seen. Surfing around Jensen's website showed transofrmers upwards of $200 ea, I think they were for tube outputs or something.

So doing the math, it's about 10 times more costly then a CMoy, and it would probably sound better to simply build a Dynalo.

 

[Clutz]

Quote:

Originally Posted by rickcr42 the premise is a correct one but you are attacking it all wrong ! Just use a high current output opamp like the Analog Devices AD8397 as a single stage headphone amp with the gain set at no more than +6dB. Not only are output transformers expensive but those with a 32 ohm output VERY expensive and to add them to a simple opamp just to drive headphones not a good cost vs. performance ratio.

Hello Rickr42,

Thank you for answering my question. I've seen you be an advocate for the use of transformers in some cases, so I appreciate it! I'll take a look at the data sheet for the AD8397 and see if I can figure something out.

Thanks!
Clutz

 

[Clutz]

Quote:

Originally Posted by Garbz weeeee doubel post big guy, you're a bit trigger happy By expensive since no one's said it means it cost about $150USD for the transformers. And these are in one of the cheapest implimentations that I've seen. Surfing around Jensen's website showed transofrmers upwards of $200 ea, I think they were for tube outputs or something. So doing the math, it's about 10 times more costly then a CMoy, and it would probably sound better to simply build a Dynalo.

Hey Garbz,

Thank you! You and Rick have completely answered my questions. I already have most ofa PPA put together (received the last of the parts I needed on Friday - I just have to figure out how to case it now.. I don't have a drill, let alone a drill press, let alone anywhere to use them if I had them - small apartment).

I already have two dynalo boards ready to go - so that'll be next.. and then maybe an M3..

Arghhh. I really want to get thisnew dac kit too!

 

[rickcr42]

Quote:

Rick, As a side question, do you know of any particular transformers with 32 ohm outputs that you would recommend?

Check out the Sowter (just google sowter) .Should be just about right considering all the configuration options

Quote:

I've seen you be an advocate for the use of transformers in some cases, so I appreciate it! I'll take a look at the data sheet for the AD8397 and see if I can figure something out.

My system is loaded with transformers but in this case it would be a misuse.my answer just above this one is for a triode output needing to be mated to a 32 ohm load and that is not possible with a single ended tube stage so in this case loading the output of the triode with a high impedance and with the trasnformer secondary providing the low impedance to the cans an ideal match.
With solid state this impedance "mismatch" is not there since the "actual" opamp output impedance is in the tenths of ohms and not hundreds or thousands of ohms so it becomes all about clean current delivery into the 32 ohm load for the opamp or buffer.

where do I use a transformer with solid state ?
The output of all my DACs,input to me ADC and anywhere I need a single ended to balanced line adapter or ground loop/digital ground isolation.

I also use iron as the phase splitter on PP tube amps,in my hybrid headphone amp,on the input of my class-A amp which I use it to "split" to two mono amps making a Single Ended Push -Pull Class-A amplifier (SEPP) from two identical SE Amps,on the output of my line level WOT Triode stages (WOT means With Output Transformer

)..........

Actually that means everywhere else but on the output of a simple opamp meant to drive headphones directly

 

[rickcr42]

Quote:

weeeee doubel post big guy, you're a bit trigger happy

yeah I seen the word "transformer" and got all sweaty and stuff

 

[dsavitsk]

Quote:

Originally Posted by rickcr42 Check out the Sowter (just google sowter) .Should be just about right considering all the configuration options

Is the 8665 or the 8976 (or something else) the more appropriate option? The 8976 has a primary impedence of 2K and a secondary or 18 while the 8665 (which is way less expensive) has a primary of 10K and the data sheet shows that it should work with a 30 ohm load. It does specify that it is intended to be used with a cathode follower, but perhaps Pete's design has a low enough output impedence?

 

[rickcr42]

Is P.M.s design a single ended one ?

***edit****

just took a quicky look at the low mu preamp and the output is 324 ohm roughly and is that low because of the pentode loading the low mu triode.will check the sowters and be back........................

 

[rickcr42]

8655 looks to be the best choice for your preamp which is not single ended but a MU Stage.Very close to the SRPP with some differences and for some good background check out the Tube CAD Journal SRPP Deconstructed (google again)