Adding headphone amp to vintage integrated tube amp

[mariospeedwagon]

Hi everyone,
New to the forum and the world of headphones.

My 60 year old Harman Kardon A300 integrated tube amp doesn't have a headphone jack, but it does have a pair of "TAPE OUT" RCA jacks.

The amp's manual states the signal from these jacks is a half volt. I'm trying to be a considerate neighbor and would like to be able to use my new Sennheiser HD660S with this amp so I can enjoy my records at all hours of the night. I'm shopping around for a headphone amp and before I get too far, I'd like to know if this half-volt output going to be problematic (seems low to me).

Issue #2 is that TAPE OUT bypasses the tone controls of the amp. When I listen with the speakers (Klipsch Forte) I usually have the contour switch engaged because the amp is only 15 watts per channel and somewhat lacking in the bass department. This switch is basically bass boost/Loudness button and really makes this amp wonderful.

Any advice on what I should use to amplify this wimpy 1/2 volt signal that needs some low-end boost? I'd like to keep it under $500

 

[pbui44]

The TAPE-OUT on your Harman Kardon A300 is tuned for tape sources, so I think that you should get a headphone amp that will also act as a preamp to your A300. With that said, this headphone amp will also work great as a pre-amp:

https://www.headamp.com/products/gilmore-lite-mk2?variant=34607281995941

 

[mariospeedwagon]

The TAPE-OUT on your Harman Kardon A300 is tuned for tape sources, so I think that you should get a headphone amp that will also act as a preamp to your A300. With that said, this headphone amp will also work great as a pre-amp:

https://www.headamp.com/products/gilmore-lite-mk2?variant=34607281995941

Thanks for the response and info.

So if I need an amp that also acts as a pre-amp, should I really be paying close attention to the Gain spec as I shop around? The gilmore you linked to says it provides 7.5X gain (16.5 dB). I'm seriously considering the Monolith Liquid Platinum, especially with it's balanced XLR output. The specs on that one show 5X gain (14 dB). Would you think this would be a good match given that my Senn 660s are efficient (SPL = 104 dB @ 1V) and have an impedance of 150 ohm?

 

[pbui44]

Thanks for the response and info.

So if I need an amp that also acts as a pre-amp, should I really be paying close attention to the Gain spec as I shop around? The gilmore you linked to says it provides 7.5X gain (16.5 dB). I'm seriously considering the Monolith Liquid Platinum, especially with it's balanced XLR output. The specs on that one show 5X gain (14 dB). Would you think this would be a good match given that my Senn 660s are efficient (SPL = 104 dB @ 1V) and have an impedance of 150 ohm?

Wow...you are basically telling me that you are gung-ho about using the Tape-Out, even if it is intended for tape sources.

Pay less attention to the gain levels and more towards the watt RMS @ x ohm. Typically, a headphone amp with a straight source will be sensitive to gain, but since you are adamant on using the Tape Out, watt RMS @ x ohm is important. The Liquid Platinum has 3.6W RMS @ 50 ohms and goes for $800, while this Schiit Magnius has 3.2W RMS @ 50 ohms and goes for $200:

https://www.schiit.com/products/magnius

 

[mariospeedwagon]

Wow...you are basically telling me that you are gung-ho about using the Tape-Out, even if it is intended for tape sources.

Unfortunately, Tape-Out is my only option. I'm new to headphones (obviously) so right now my preference is still to listen via loudspeakers and I love my A300. I'm just looking to add headphone functionality to my rig so I can be a good neighbor and keep the noise down after 10pm. I got a good deal on the HD660S, so I thought I'd take a plunge into the headphone world. Hopefully it's possible, even via Tape Out, to get a headphone setup that sounds as good as my loudspeaker setup. I do have a DAC for when I want to listen to Qobuz, but my listening is still predominately done with my turntable and amp.

Whatever amp I buy, it's likely that I'll experiment with hooking it up directly to my DAC (Black Ice) when I want to listen to Qobuz at night---just to keep the signal chain shorter. In that case it might be nice to have something with a low/high gain switch (like the Schiit Asgard).

I appreciate your help. The research continues.....

 

[pbui44]

The Schiit Magnius also has two gain levels and the gain switch is on the front with the SE/balanced input switch.

 

[tomb]

Unfortunately, Tape-Out is my only option. I'm new to headphones (obviously) so right now my preference is still to listen via loudspeakers and I love my A300. I'm just looking to add headphone functionality to my rig so I can be a good neighbor and keep the noise down after 10pm. I got a good deal on the HD660S, so I thought I'd take a plunge into the headphone world. Hopefully it's possible, even via Tape Out, to get a headphone setup that sounds as good as my loudspeaker setup. I do have a DAC for when I want to listen to Qobuz, but my listening is still predominately done with my turntable and amp.

Whatever amp I buy, it's likely that I'll experiment with hooking it up directly to my DAC (Black Ice) when I want to listen to Qobuz at night---just to keep the signal chain shorter. In that case it might be nice to have something with a low/high gain switch (like the Schiit Asgard).

I appreciate your help. The research continues.....

Tape Out is the same as any other non-amplified output from a pre-amp. The output level is essentially the same as any other source. You can happily connect the headphone amp, just as if it were any other source - DAC, CD player, tuner, etc.

Volts can mean many things - peak, peak-to-peak, rms, etc. The typical line-level voltages for consumer audio is as follows:

Consumer audio

−10 dBV​

0.316 Vrms​

0.447 Vpeak​

0.894 Vpp​

Depending on what your Harmon Kardon integrated amplifier is quoting, you could be well in the norm.

The difference is that your Integrated Amp will have its own volume control, so you'll have to play around with matching the volume on the headphone amplifier. The first thing to try might be 100% volume on the headphone amplifier, starting with 0% on your Integrated Amp. You need to be cautious that you don't have any connected load (headphones) present when doing this, however, because the gains could be mis-matched. A little careful trial and error will let you discover a happy medium.

Not sure what the other poster is talking about, but let me repeat: Tape Out is a line-level source output from any preamp/integrated amp/receiver. Tape Input is no different than an "AUX" input from a modern component. Tape Output works the same way. It's good to go with a headphone amplifier.

 

[mariospeedwagon]

The difference is that your Integrated Amp will have its own volume control, so you'll have to play around with matching the volume on the headphone amplifier.

According to the HK manual, the Tape Out is unaffected by the integrated amp's volume controls. So I'll just turn the HK all the way down and rely on the headphone amp's volume controls.

Regarding the HK's wimpy bass---do you think this will be less of an issue after the signal gets boosted by a headphone amp? Or should I try something like the Schiit Loki? I suppose I can do a little trial and error and pick up an amp first and then add an EQ later if necessary.

Thanks. I appreciate your response.

 

[tomb]

According to the HK manual, the Tape Out is unaffected by the integrated amp's volume controls. So I'll just turn the HK all the way down and rely on the headphone amp's volume controls.

Regarding the HK's wimpy bass---do you think this will be less of an issue after the signal gets boosted by a headphone amp? Or should I try something like the Schiit Loki? I suppose I can do a little trial and error and pick up an amp first and then add an EQ later if necessary.

Thanks. I appreciate your response.

That's good - I couldn't remember if the Tape Monitor was volume-controlled or not, so that's best all around!

As for your wimpy bass, I suspect you have worn out capacitors in your amp's circuitry. That will cause a bass loss. Chances are, the line-level outputs are not affected by this, only your speaker outputs, but I'm just guessing not knowing the specifics of your equipment. When you boost the bass using tone controls, you're artificially compensating for the bass loss that's already there - either in the speakers or the amp - but you already knew that.

30 years is usually the life limit for electrolytic capacitors. If yours is 60, I'm surprised you're not also getting hum through the speakers, too. Again, my guess is this will not affect the line-level outputs, so your headphone amp should be fine. Best scenario is always to feed a headphone amp directly with a DAC or other source, but if this is what you need, I'd give it a try, first.

 

[mariospeedwagon]

If yours is 60, I'm surprised you're not also getting hum through the speakers, too

My apologies--I was a little misleading in my original post. The amp is 60 years old but the caps are only about 5. It was restored and works wonderfully. No hum to speak of.

I did a little experiment lasts night with the Tape-Out. I plugged into my MacBook to record a few tracks from an LP (The Band's "Big Pink") with Audacity. Playback of the recording from the Macbook via B+W PX headphones sounded really good. But the bass isn't great (I hope I don't sound like a bass-head. I'm really not). The signal in this preamp stage (also pre-tone controls) must be really flat and dependent on the output stage and tone controls for it's oomph. This amp is only 15 WPC but with the Forte speakers it's capable of sounding enormous even at low volume.

 

[CJG888]

I would propose a different approach.

First of all, I would attend to the general health of your (very special) amp. All electrolytic capacitors should probably be replaced, and some of those old (good-sounding) carbon resistors may also be out of spec. Once this has been attended to, the sound will be transformed. It will sound like someone put a Hemi under the hood...

While you’re at it, check that all the tubes are good. Decent NOS tubes for this amp (6V6?) shouldn’t break the bank.

Now your speaker system should have enough bass without the loudness control!

With a high quality switching box and an appropriate adapter (resistor network), you should be able to drive your HD660s from the speaker terminals. As a safety precaution, it may make sense to use a balanced XLR connection with separate left and right grounds.

 

[mariospeedwagon]

First of all, I would attend to the general health of your (very special) amp.

I should have mentioned this originally, but the amp was restored about 5 years ago by a highly-regarded HK expert. You're right--its a very special amp and I wish I could live somewhere where I don't have to be a good neighbor after 10pm and use headphones. Such is life in New York City.....

While you’re at it, check that all the tubes are good. Decent NOS tubes for this amp (6V6?) shouldn’t break the bank.

Tubes in the preamp stage (4x 12ax7 and 1x 12au7) are NOS Mullards and Telefunkens. I have other NOS options that I roll in from time to time but most of the time I use that array. It uses 4 6V6 in the output stage and I use NOS RCA's or the original HK 7408 that came with the amp and still test strong. I've done a fair amount of tube rolling since acquiring this amp and I'm really in love with what comes out of the loudspeakers, but that warmth doesn't survive the trip through the Tape-Out. Everything else does (soundstage, timbre, low noise, etc).

I believe what is happening is that the Tape Out signal is going to Tape Out right after the preamp stage (V1-V4) and before it gets to the rectifier (V5) and output tubes (V6-V9), so I'm hopeful that once this signal encounters amplification, via a Headphone amp, the bass will (magically?) reappear.

 

[mariospeedwagon]

you should be able to drive your HD660s from the speaker terminals

Would this involve a lot of changing back and forth between speakers/headphones?---because these terminals are not much fun to deal with.

or would that problem be solved by the "switching box" that you mentioned?

 

[CJG888]

That would be taken care of by the switching box. It needs to be a good one, though, so as not to degrade the sound (expect to pay at least $100 or so).

Which generation of Fortes are you using?

 

[CJG888]

If your tape out is really only putting out 500mV or so (presumably for the sensitive tube input stages of contemporaneous open-reel decks), this will barely be enough to drive a desktop headphone amp properly. An iPod puts out more than that. Modern “tape outs” are effectively line outputs (@ 2V).

If you insist on using the tape out, you may need an additional gain stage. There may also be an impedance mismatch.