[Steve Eddy]

I was at 6 Moons earlier reading the Music First Audio Tap review in the course of which I followed the link to the Burson Audio Buffer review and from there to Burson Audio's website.

The "problem" according to Burson is the high output impedance of mid-priced CD players. According to Burson:

Most mid range CD players priced below $2000 (even some over) will have a very simple output stage with nothing more than a pair of Op-amps. That will result in higher output impedance (typically 150 ohms to 500 ohms that limits Macro and Micro dynamics.)

Their solution is the Burson Audio Buffer which they say:

...is able to reduce the output impedance of a typical CD player to a range between 15-30 ohms. That will translate to more micro and macro dynamics; better control at the lower end and effortless at the top.

If you stop and give it just a little bit of thought, the buffer really doesn't reduce the output impedance of the CD player. The output impedance of the CD player remains the same as it was before. And if its high output impedance was indeed responsible for limiting dynamics, then dynamics will have already been limited by the time the signal is fed to the buffer in which case the buffer can do nothing to restore that which has already been lost.

The implication is made that it's the opamps themselves which are responsible for the high output impedance of the CD player. This isn't true. The output impedance of a typical opamp can be as low or even lower than that of the output impedance of the Burson buffer.

What accounts for the common 150-500 ohm output impedances are the resistors used in series with the opamp's outputs to assure that the opamp sees an approrpriately high impedance at RF frequencies to prevent instability and oscillation when connected to a capacitive load like a cable.

But you can have your cake and eat it too.

Instead of a single, high value resistor, you can use a parallel resistor/inductor. At audio frequencies, its impedance will be just a fraction of an ohm, allowing you to take full advantage of the opamp's low output impedance, but at RF frequencies, its impedance increases sufficiently to keep it isolated from the capacitive load of the cable.

Jensen Transformers already sells a ready-made RL network designed specifically for this purpose. It's their JT-OLI-3. It sells for $4.65 in 1-3 quantities.

An effective and inexpensive way to hotrod the output most any CD player with opamp outputs.

se

[Daygloworange]

I know in pro audio, stand alone microphone preamps ( which are the norm nowadays) have input ohm selector switches to change the impedance level the mic sees at the pre amp input, which has a significant effect on how a dynamic mic will sound.

If their claim is valid, it seems like a simple solution to get the correct impedance match to not negatively impact dynamics and so on, why hasn't anyone else caught on to this and corrected it a long time ago?

Or is it not quite as simple as it sounds?

 

[NealH]

Assuming the op-amp topology includes fixed resistors inserted in series with the ouput of the op-amps, then I would agree that substituting your referenced LR circuit for this fixed resistor should reduce the output Z while at the same time alleviating the "dyamics" issue - assuming the op-amp can deliver the current demand of the cable/amp input requirement.  If not then dynamics would still suffer - though perhaps not to the same extent.

Regardless, adding a low Z buffer to an op-amp that has additional resistance placed on it's output will lower the  Z of the player's output circuit.  An op-amp circuit with fixed resistance (150-600 ohms) added to it's output will likely not inccur the same limitations when driving the input of a buffer amp as when it is driving a cable/amplifier input circuit.  While this limition may or may not exist depending on what the actual cable/amp effective input Z looks like, the CD's true output Z has been lowered by the use of the buffer (assuming resistance isn't added here).      

[G Georgopoulos]

If you stop and give it just a little bit of thought, the buffer really doesn't reduce the output impedance of the CD player. The output impedance of the CD player remains the same as it was before.
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no most buffers do reduce impendance
i cant think of any other use of buffers

regards
George

[Steve Eddy]

I know in pro audio, stand alone microphone preamps ( which are the norm nowadays) have input ohm selector switches to change the impedance level the mic sees at the pre amp input, which has a significant effect on how a dynamic mic will sound.

If their claim is valid, it seems like a simple solution to get the correct impedance match to not negatively impact dynamics and so on, why hasn't anyone else caught on to this and corrected it a long time ago?

Or is it not quite as simple as it sounds?

I don't think it's so much it being not as simple as it sounds, but rather more a case of apples and oranges.

A dynamic microphone is an electromechanical transducer, and rather like a loudspeaker or a phono cartridge, is fundamentally a resonant system. In resonant systems, source and load impedances can have significant effects on their dynamic behavior.

Take a simple loudspeaker driver for example. It's basic resonant behavior is summed up in its total Q parameter, Qts. If its Qts is less than 0.5, it's said to be overdamped and its low frequency response will be rather rolled off. If its Qts is greater than 0.5, it's said to be underdamped and its low frequency response will be characterized by a certain amount of peaking, along with ringing and overshoot. If its Qts is 0.5, it's said to be critically damped and will have the best low frequency response without any ringing or overshoot.

However the driver's Qts assumes that it's being driven from a source impedance of zero ohms. If you drive it from an increasing source impedance, Q will increase and the low frequency behavior of the driver will change.

But electronic interfaces aren't resonant systems. Well, they're not fundamentally resonant systems. There are parasitic elements which can combine to create resonances, but that's another story. Bottom line is, genreally speaking we simply want a low source impedance driving a high input impedance. That's because we're most concerned with transmitting signal in the form of voltage.

The output impedance of the source forms a voltage divider with the input impedance of the load and the higher the source impedance is relative to the load, the more loss of signal there will be. If the source impedance is the same as the load impedance, you end up with a 6dB loss of signal.

The output impedance also has rammifications with respect to high frequency rolloff in combination with the capacitance of the cable you're driving.

Anyway, hope this helps.

se

[G Georgopoulos]

Bottom line is, genreally speaking we simply want a low source impedance driving a high input impedance. That's because we're most concerned with transmitting signal in the form of voltage.
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i couldnt put it better myself

cheers

[Daygloworange]

Steve Eddy,

Thanks for the detailed explanation, its beyond my grasp of electronics. I'm a Recording engineer with a basic understanding of electronics. I know how to use gear to achieve certain sounds, I understand the functions of equipment but not the in depth way the parts work in order to achieve them.

As I've gotten into the "2 channel" audiophile thing, I've expanded my "general" knowledge quite a bit, but still light years away from a lot of people.

As I've always understood it, the balanced cables that we run are less sensitive to resistance due to long lengths of cable. I don't know how picky people get as to specs. The recording industry is a "follow the leader" type of thing. Who ever are the BIG names dictate what gear everyone else SHOULD be using. Very different from the audiophile community.

Cheers

[Steve Eddy]

no most buffers do reduce impendance
i cant think of any other use of buffers

I think you're missing the distinction here.

By the output impedance of the CD player, I mean literally the output impedance of the CD player, not the output impedance of the buffer. While the CD player is connected to the buffer and the buffer has a lower output impedance than the CD player, the output impedance of the CD player itself is still the same as what it was before. That hasn't changed.

You following me?

Burson says that it's the high output impedance of the CD player that's limiting dynamics. But if that's the case, then as I said, since the output impedance of the CD player itself hasn't changed, then the dynamics will have already been limited at the CD player's output and there's nothing the buffer can do at that point. It can't add dynamics that weren't already there at its input.

And speaking of input, let me frame this a bit differently.

What difference does it make if your CD player is driving the input of the Burson buffer or the input of your preamp? Burson doesn't many any particular claims about the input impedance of their buffer. In fact they don't say anything at all about it. They only speak of its low output impedance.

So let's say that the input impedance of the Burson buffer is 50k ohms. And the input impedance of your preamp is also 50k ohms. What difference does that make to your CD player? What's it matter whether it's driving one 50k ohm load versus some other 50k ohm load?

Nothing that I can see. And in either case, you still have to drive the cable that's connecting the CD player to either the Burson buffer or the preamp.

So you see what I'm saying now?

And the point of my post is that for a few dollars, the CD player's output impedance can be as low or lower than the output impedance of the buffer.

se

[amplifierguru]

Yeah, what he said!

[Steve Eddy]

Yeah, what he said!

I disrespectfully disagree.

[Russell Dawkins]

Instead of a single, high value resistor, you can use a parallel resistor/inductor. At audio frequencies, its impedance will be just a fraction of an ohm, allowing you to take full advantage of the opamp's low output impedance, but at RF frequencies, its impedance increases sufficiently to keep it isolated from the capacitive load of the cable.

Jensen Transformers already sells a ready-made RL network designed specifically for this purpose. It's their JT-OLI-3. It sells for $4.65 in 1-3 quantities.

An effective and inexpensive way to hotrod the output most any CD player with opamp outputs.

se

thanks for that, se, that's the juiciest tip I've read in a long time!
Russell (another recording engineer)

[Steve Eddy]

Thanks for the detailed explanation, its beyond my grasp of electronics. I'm a Recording engineer with a basic understanding of electronics. I know how to use gear to achieve certain sounds, I understand the functions of equipment but not the in depth way the parts work in order to achieve them.

I don't know that that's so bad. You know what they say about not wanting to see how sausage is made.

As I've gotten into the "2 channel" audiophile thing, I've expanded my "general" knowledge quite a bit, but still light years away from a lot of people.

Don't feel bad. We're really not much different in that respect. I'm hardly any expert.

As I've always understood it, the balanced cables that we run are less sensitive to resistance due to long lengths of cable. I don't know how picky people get as to specs. The recording industry is a "follow the leader" type of thing. Who ever are the BIG names dictate what gear everyone else SHOULD be using. Very different from the audiophile community.

Oh I dunno. I think there's a fair amount of "follow the leader" mentality in the audiophile community. Though I think it tends to be more lateral than vertical.

se

[Steve Eddy]

thanks for that, se, that's the juiciest tip I've read in a long time!

Was that juice? I was wondering what that sticky stuff was, but was afraid I might not want to know the answer.

Anyway, you're welcome. Hope it works well for you!

se

[NewBuyer]

...Burson doesn't many any particular claims about the input impedance of their buffer. In fact they don't say anything at all about it. They only speak of its low output impedance.

So let's say that the input impedance of the Burson buffer is 50k ohms...

Just for the record, the Burson Buffer has an input impedance of 550K.

I wish I was able to technically explain exactly why/how this discrete Burson Buffer makes such a very nice sound improvement in my system, but it definitely does what it claims to do, and is an extremely high-quality device IMO.

If that little Jensen piece does all the same tricks just as well, then I would be really surprised, and would wish all digital players/converters will listen to your advice Steve! 

[PaulHilgeman]

...Burson doesn't many any particular claims about the input impedance of their buffer. In fact they don't say anything at all about it. They only speak of its low output impedance.

So let's say that the input impedance of the Burson buffer is 50k ohms...

Just for the record, the Burson Buffer has an input impedance of 550K.

I wish I was able to technically explain exactly why/how this discrete Burson Buffer makes such a very nice sound improvement in my system, but it definitely does what it claims to do, and is an extremely high-quality device IMO.

If that little Jensen piece does all the same tricks just as well, then I would be really surprised, and would wish all digital players/converters will listen to your advice Steve! 

While the device had merit, it can only be realized (or most realized) with high output impedance CD or Pre and low input impedance amplifiers.

As to the particular output series L//R philosophy, great idea, but for some reason designers shy away from it, I actually used it in some filters I designed a while back, worked perfectly the sound stayed shockingly similar with high input impedance amps, low input impedance amps, long cables etc.  It let the output stage do what was required. 

550K ohms is a nice input impedance, and probably in many cases, it can lower the distortion of a previous gain/buffer stage, especially when compared to driving an amp with a 5K input impedance.

Always remember however, that every piece of the chain has a sonic signature, and proper system design asside, you are simply adding another link into an already long chain with this device.  A better solution would be to keep cables short, use high imput impedance amplifiers, and low output impedance shources.

-Paul

[Bob Reynolds]

Jensen Transformers already sells a ready-made RL network designed specifically for this purpose. It's their JT-OLI-3. It sells for $4.65 in 1-3 quantities.

An effective and inexpensive way to hotrod the output most any CD player with opamp outputs.

Thanks Steve. I'm a big fan of Jensen products. I use several in my main system, but I was unaware of the product you cite.

[Gordy]

Agreed!  I like keeping a Jensen line iso transformer  http://www.jensentransformers.com/ci2rr.html  around for those rare times I run into a ground loop.  Vastly superior to lifting / removing ground plugs IMHO.

[Steve Eddy]

Just for the record, the Burson Buffer has an input impedance of 550K.

Thanks. That's certainly higher than typical, but not exactly unheard of, parcularly with tube gear.

I wish I was able to technically explain exactly why/how this discrete Burson Buffer makes such a very nice sound improvement in my system, but it definitely does what it claims to do, and is an extremely high-quality device IMO.

And that's great. I was never trying to insinuate that it's a bad product. I was just speaking to that bit of uh... "logic" on the website about the output impedance of CD players. I'm weird that way. I completely miss that it's Promitheus and not Prometheus, but things like that just stick out like a sore thumb to me.

If that little Jensen piece does all the same tricks just as well, then I would be really surprised, and would wish all digital players/converters will listen to your advice Steve! 

Hehehe. Well the OLI can lower the output impedance that's just using a series resistor on its output, but it certainly can't do everything that the Burson does. I mean, if nothing else, the Burson has 6dB of voltage gain. Something you ain't gonna get with a resistor and an inductor.

se

[Steve Eddy]

Thanks Steve. I'm a big fan of Jensen products. I use several in my main system, but I was unaware of the product you cite.

Yeah, they sort of keep it hidden. It's on the line output transformer page, at the very bottom.

I only remember it because years ago I received their big hardcopy binder that had all their datasheets, app notes, etc. which I flipped through cover to cover more than once.

And even there, the OLI datasheet was on the last page of the line output transformer page.

se

[Steve Eddy]

Agreed!  I like keeping a Jensen line iso transformer  http://www.jensentransformers.com/ci2rr.html  around for those rare times I run into a ground loop.  Vastly superior to lifting / removing ground plugs IMHO.

Not to mention safer. 

se