PV-5 as headphone amplifier?

[tubesound]

Hi everyone! My first post here, but I've been reading this forum over a year now

I was thinking of connecting some headphones directly to the output of my PV-5 with an RCA adapter and seeing how it sounds.

How does the maximum power output of the PV-5 vary with headphone impedance?

And what is the precise output impedance of the PV-5? The manual says "less than 200 ohms", but how much less?

How can I make headphones see a lower output impedance from the PV-5? Maybe using a resistor built into an adapter between the RCA out and the headphones?

I was also wondering, since I'm not using the phono stage, are the tubes in the PV-5's phono stage losing life even when the PV-5 is functioning only as a line stage? If so, which tubes are in the phono stage, so I can take them out and put them aside?

I would appreciate any helpful comments.

Thanks
Frank

[admin]

Hi Frank.
First, welcome to CJO. Great to have you with us.

As to your questions. I suppose it can be done but the better question is should it be done? The typical impedance of an amplifier is on the order of 10,000 ohms where as headphones are typically much lower (even down to double digits). Clearly the PV-5 was not designed to drive headphones directly. If you are really looking to get high quality CJ sound from your headphones, then I would strongly recommend the CJ HVA-1. This is what you are looking for. If that is out of budget, I would recommend one of the headphone amplifiers from Schiit. They have inexpensive products and they are made here in the US. I own two of their units myself. Lots of bang for your buck.

I think the phono stage is always on (when the unit is on). I would not removed tubes in a circuit path that were designed to be active. If you are worried about cost of the tubes and you don't use the phono stage. You can get some really cheap replacements (some cheap Chinese non-audiophile tubes) for a few bucks and just run them in there instead of your higher quality ones.

[AnotherJohnson]

I agree with admin.
At one time Radio Shack sold a junction box to run headphone from the speaker outputs in an amp. But running off a preamp without a dedicated headphone jack is not going be be very good.

If you want to add headphones, another option would be to buy a source with a built in headphone jack.

[tubesound]

Hi to both of you, and thanks for taking the time to reply! While I appreciate your answers, I'm still left wondering.

I already have a headphone amplifier, and I know preamps aren't designed to drive headphones directly. Nonetheless I am curious, how well could one do, if one had to, with the PV-5 driving headphones directly?

I read that for headphones "Commonly a damping factor greater than 2.5:1 and nearer to 8:1 is considered optimal (headphone impedance 2.5x to 8x greater than the source impedance)." See https://www.headphonesty.com/2019/04/he ... mystified/

There are higher impedance headphones; for example there was a 2000 ohm version of the Sennheiser HD424, and I've even heard about vintage 5000 ohm orthodynamic headphones.

So I wonder:

1. What is the exact output impedance of the PV-5 (as a function of frequency)?

2. What is the maximum power output of the PV-5 into 600 ohm headphones (such as the Beyerdynamic T1)?

There are impedance adapters for headphones, like this one:
https://www.ebay.com/itm/Raphaelite-B10 ... 3270638994
and I wonder if that might lower the output impedance sufficienctly.

Out of curiosity, if the tubes in the PV-5 phono stage are all dead, but the tubes in the line stage portion are just fine, is there any harm in continuing to use it as a line stage only? If so, can the phono stage portion of the circuit be fully disconnected, so that it would be capable of functioning only as a line stage?

Thanks again for your time and sharing your knowledge!

[admin]

In all honesty I am not knowledgeable enough about headphone vs amplifier comparisons when it comes to preamp output signal management.

I know this. Headphones typically have a lot less impendence than amplifiers. This can be on orders of magnitude difference.

I guess it begs the question, what is the purpose of this endeavor? Do you think you will get better sound out of a vastly mismatched preamp output than a headphone amplifier? I guess you can plug it in and try it?

As for the phono side. It's clearly meant to be on while the unit is on. Buy disconnecting or having faulty components, you risk potential damage. Who really knows exactly what would happen to the circuit if one particular tube is blown. I know on my CJ ART amps, if an output tube is blown, it also blows the fuse. That's not so great as it's telling me the current goes above specs when that happens. Again, you could probably source the cheapest available tubes for under $30 total and just have the security that the unit will function to spec vs taking a risk of damage.

[AnotherJohnson]

This is starting to look like a troll.

There are four places to get your answer.

1. Convince CJ to give you their ancient intellectual property.

2. Call Bob Carver and see if he’s got it from his days trying to duplicate the sound of other people’s intellectual property.

3. Inquire with the Chinese companies who have reverse engineered many CJ and Krell products so as to sell counterfeits on eBay.

4. Do your own research at the test bench rather than with google.

I don’t mean for this to sound unsympathetic... but certainly you could cobble together a headphone amp and kluge it onto the PV5. Personally, out of respect for a really great, even legendary, phono equipped preamp, I hope that you’ll sell it to someone who would restore it.

As for the phono tubes, they’re not marked in the manual as phono, but they may be marked on the schematic.

A phono preamp circuit would usually be close to the phono inputs and far from the power supply.

Pull them one at a time ... if the missing tube doesn’t affect the line stage sound, leave it out and move on. If it does, put it back in and move on. Turn the unit off before installing or removing tubes. There are lethal voltages inside, even when unplugged... so don’t poke around inside unless you actually understand how to avoid becoming the path to ground for energy stored in a capacitor.

If I have a chance to look at the schematic I will. Roberto may know from his service work.

I’ve probably shared too much.

[AnotherJohnson]

From the schematic I found, it looks like V1, V2, V3 & V4 are only used by the phono section. It looks like you could pull these and not affect the line level inputs.

FWIW in V1 and V2, both triodes are used by the same phono channel, V1 left, V2 right.

Half of V3 is used by each phono channel, and the same is true of V4.

If you look at the owner’s manual tube diagram, you can see this is the practical layout.

Maybe Roberto would confirm or correct.

[tubesound]

Hi admin, I guess the purpose of the endeavor is to hear how the PV-5 sounds on headphones directly, without going through another amplifier. Just curious to hear the sound of the PV-5 more purely, unimpeded. The CJ headphone amp would be great to hear, but that's beyond my budget.

A source impedance that is higher than ideal (relative to the headphone's impedance curve) leads to bass roll off (the bass doesn't extend down as low), but maybe I can put a resistor in a cable between the headphones and the RCA outputs to lower the output impedance enough. Hopefully that won't lower the output power too much, but still worth trying.

Thanks AnotherJohnson for your suggestion to remove the V1...V4 tubes from the phono stage.

[AnotherJohnson]

It is hard to talk about the sound of a preamp because a preamp is intended to drive an amp, not speakers. What you hear will be load dependent and the mechanical AND electrical load of a speaker will always be a mismatch compared to the electrical load of a properly matched amp.

There are some DYI forums inhabited by those committed to “going where no man has bothered to go before.” You might find some like minds there, and some may have better arguments for or against than most here.

I’ve been hoping that Roberto might comment. He may have had customer interest in this sort of idea ... but then again ...

If I were insistent on trying this, I’d get rid of the rca output jacks and replace each with a pair of output posts. In fact, in order to avoid buggaring up the PV5, I’d just build a fused outboard box with female rca inputs on one end, resistance loads chosen for desired impedance in parallel or series as needed running through the box, to a stereo headphone jack on the other side of the box. The resisters would not have to be super power dissipators, but there will be some power dissipation. You can roughly estimate what resistance values you want ... lower than the headphone impedance, put them in parallel. Higher, put them in series.

You don’t need to know the complex impedance over the entire 20kHz range ... just try a few things. But don’t forget the fusing to protect the PV5.

This approach, with the fuses separate box connected to the PV5 as though it were an amp, protects you and the PV5 to some extent. At least you can sell the PV5 later after you’ve learned as much as it can teach you.



My emotions are torn between admiration for your explorer’s mind set, and sadness over the molestation of a legendary component.

If the PV5 can give its last life to help someone learn something, perhaps that is enough.

[AnotherJohnson]

An anecdote from a CJ legend who will remain unidentified.

The source, preamp, amp, and speakers are an organic single machine that creates sound in a room. They perform together. If one is out of kilter, the sound will not be as intended by the media being played on the source.

Back to my thoughts. People talk about house sound, signature sound, amp sound ... it all means little unless you’re talking about sound of a component in the context of a room and system.

[AnotherJohnson]

Here’s a nice OTL design with RCA (unbalanced) inputs like CJ prefers.

https://www.schiit.com/products/valhalla-1

This family, Schiit, was suggested by admin too.

You could buy one of these, take it apart, and learn quite a bit about output transformerless designs. It might be applicable to molesting née modifying the PV5. You might be able to abandon the phono section and use V1 and V2 as gain stages, cut out the RIAA equalization, drill a hole in the face plate for a headphone jack, etc.

My favorite OTL guy was Julius Futterman. I remember when Harvey Rosenberg acquired NYAL. That was another product line that I had as a dealer back in the old days. You would call Harvey on the phone and he would answer “You are now speaking to the voice of god.” He was quite a showman.

When Bob Carver was able to closely duplicate the sound of a pair of CJ monoblocks by his experimental reverse engineering technique in a widely followed Stereophile challenge (he basically determined the transfer function of the amps between the preamp and speakers and then duplicated it in a very small, light box), most were impressed, but Harvey was indignant. He really tore into the whole concept of stealing another man’s amp performance and advised that those in the industry should break their own ground. Time has shown that Carver was ahead of his time.

If you’ve never read about the challenge from 1985, Stereophile appears to have rerun the article in 2009.

You might enjoy it.

https://www.stereophile.com/content/carver-challenge

[tubesound]

These are stimulating suggestions!

AnotherJohnson, my first experiment would be a fused outboard box as you suggest. It would be even better to have the box's output impedance continuously adjustable with a knob

I completely agree that you always listen to a system (which includes the room, or in this case the headphones) as a whole.

AnotherJohnson, which DIY forums (diyaudio.com?) would you suggest? I have read about The Carver Challange

I have great respect for the design of the PV-5. I somewhat hesitate to mutilate the phono stage (which I have no use for), but converting it to a headphone amplification stage (with as little mutilation to the circuit as possible) is a very intriguing idea. I would want to do so in a way that would still allow the PV-5 to function sometimes as just a line-stage. Why not connect a pair of the preamp's RCA outputs to the RCA phono input, and convert the phono stage to a headphone amplifier stage (going to a headphone jack drilled into the top of the PV-5), rather than internally wiring the preamp output to the old-phono-stage-now-headphone-amplification-stage? If going this route, it would be nice to be able to power on/off the headphone stage independently of the line stage, so I would install a separate power switch for the headphone stage.

The real problem will be deciding how best to modify the phono stage circuit.

[AnotherJohnson]

FWIW, Peter Lederman uses a rheostat on his adjustable load phono preamps. Most high end gear these days acknowledges that rheostats/potentiometers have a sound signature. CJ avoids them.

As for diy forums, if you google “tube gear modifications,” stuff comes up. You also find guys at Steve Hoffman’s forum, Audiogon, and nearly every guitar amp forum on the planet. I can’t recommend any of these because I don’t have interest enough to judge them.

I did google “12AX7 headphone amp schematic” and found all sorts of stuff.

Good luck with your project.

[tubesound]

Thanks!

What type of negative feedback (and how much?) does the PV-5 line-stage circuit use?

[AnotherJohnson]

According to a review of the PV11 in a 1991 issue of Stereophile, the PV5 had “about 30 dB of negative feedback).

[bschott]

One thing to consider when hooking headphones up to any tube amp is whether the grounds of both channels are common or separate. Usually a stereo amp with two channels has a common ground between L and R output. However, in systems with monoblock amps the grounds between L and R channel are separate and are meant to always not connected together. Doing so can damage the output transformers and possibly other components in the amps. That is one reason for balanced headphones that have separate grounds. I"m not sure if connecting the L and R grounds in the PV5 could cause any damage if you use headphones with a common ground. But hey - hook it up, crank it up, and if it sounds good and doesn't catch fire then you're good to go.

[tubesound]

Thanks bschott!

I hope someone will chime in with suggestions about how best to alter the phono circuit to turn it into a headphone amplifier.

[plurn]

...
So I wonder:

1. What is the exact output impedance of the PV-5 (as a function of frequency)?

2. What is the maximum power output of the PV-5 into 600 ohm headphones (such as the Beyerdynamic T1)?
...

For 1, Realistically, you are not going to get that information from anyone though you can try and measure that yourself. Do an internet search for "how to measure output impedance". An example:

http://www.zen22142.zen.co.uk/Theory/inzoz.htm

That might look complicated at first look, but I think it is probably not - play a frequency (eg 1kz sine wave) through your preamp, and measure the AC voltage at the output on one channel with a multimeter, then put a resistor across the output and measure the AC voltage again, then do a calculation with those two values to work out the output impedance at that frequency. Then repeat for other frequencies to cover the audible range. Maybe stick the results in a spreadsheet and graph it.

Here is a calculator that will do the calculation for you once you measure the values.
http://www.sengpielaudio.com/calculator-InputOutputImpedance.htm

You might have to try it at different volume levels on the PV-5 though as I have seen some reviews of cj gear where output impedance at certain frequency ranges changes with volume setting. Perhaps just make sure you set the volume at about the expected listening level so you don't overcomplicate things.

For 2, the PV-5 manual says "Output: maximum 25v rated at 2.5v". Though I expect at anywhere near 25V it would be heavily distorted and not flat frequency response. Anyway, using Ohms law (see https://ohmslawcalculator.com/ohms-law-calculator ) at 2.5V into 600 ohm gives 0.01042 watts. 25V into 600 Ohms gives 1.04167 watts. However as you are probably aware it is not that simple - the PV-5 probably can not drive that load cleanly, and the headphones will not be exact 600 Ohms across the whole frequency range, so since these are a poor match impedance wise, you are likely going to have non-flat frequency and that can happen anywhere in the frequency range. Perhaps it will sound pleasing but I don't think you could call it the pure sound of the PV-5.

Also note that the PV-5 manual mentions this:

"Outputs:
...
Output 1, Output 2 - Connect to the inputs of amplifiers with impedance of 10k- ohms or greater."

So the headphones at 600 Ohms is a poor match.

Adding a resistor as you mentioned will give an easier load to the PV-5 however you may be left with no usable power. Ohms law calculator for 2.5V into 10000 Ohms gives 0.00063 watts.

On the other hand, you could just try connecting the headphones to the preouts and see how it sounds, and/or try it with a resistor inline and see how it sounds. Perhaps with some cheaper throw away headphones to start in case something goes wrong.

I would think a proper solution is using some sort of buffer such as a headphone amplifier - even some cheap ones using appropriate opamps should be able to transparently amplify the signal is they are designed properly. Though I see why you may wish to try the preamp more direct.

[tubesound]

Thanks plurn! Those links are great.

Any advice about how best to modify the PV-5's phono circuit to turn it into a headphone amp?

Since the PV-5's line stage already has high gain, no additional gain would be needed from the new headphone circuit.