I'm currently working over a challenge I gave myself as a way to learn and practice some recently acquired skills.
I have/had an old Marantz PM151 amp with crackly channels that was given me years ago and has been sitting around doing zip - besides it is just plain ugly - the challenge is to deconstruct and rebuild it on half the original footprint using only what I find inside it (plus solder, hookup wire and pcb standoffs).
The only new material will be used in a new front to go on it (strips of marine ply cut up and glued together so that the ply laminations are visible). So far so good with the pcb being cut up into bits, various parts discarded (balance, tone controls, some in put channels), jumper wires linking the various parts to replace broken traces. The final amp will have 2 inputs, output to speakers and headphone. The aim is for something that sounds like music to be produced from the amp even if it is not hifi - I'll be happy with that (in the meantime I found why the amp was so crackly in the first place as the solder on the pins of both main amp ICs were pretty much all broken).
Question: I don't want to use resistors to step down to the headphone output (a problem that I am also addressing in my NAD C350 over in the solid state forums) and think I can use the existing phono preamp as a buffer. It is powered by a single NJM4558DD chip. Do you think this will output enough db? I'm so new to this that I don't know which spec to look for on the data sheet
Thanks and good day to all.
I'll be sure to post pics when it is all put together.
I have/had an old Marantz PM151 amp with crackly channels that was given me years ago and has been sitting around doing zip - besides it is just plain ugly - the challenge is to deconstruct and rebuild it on half the original footprint using only what I find inside it (plus solder, hookup wire and pcb standoffs).
The only new material will be used in a new front to go on it (strips of marine ply cut up and glued together so that the ply laminations are visible). So far so good with the pcb being cut up into bits, various parts discarded (balance, tone controls, some in put channels), jumper wires linking the various parts to replace broken traces. The final amp will have 2 inputs, output to speakers and headphone. The aim is for something that sounds like music to be produced from the amp even if it is not hifi - I'll be happy with that (in the meantime I found why the amp was so crackly in the first place as the solder on the pins of both main amp ICs were pretty much all broken).
Question: I don't want to use resistors to step down to the headphone output (a problem that I am also addressing in my NAD C350 over in the solid state forums) and think I can use the existing phono preamp as a buffer. It is powered by a single NJM4558DD chip. Do you think this will output enough db? I'm so new to this that I don't know which spec to look for on the data sheet
Thanks and good day to all.
I'll be sure to post pics when it is all put together.
Your receiver probably has +/- 15 volt supply already, so why not build a proper headphone amp? Look at BUF634 and LME49600 datasheets. They have high quality headphone amp circuits that use a minimum of parts. That plus a volume control and you'll have a headphone amp capable of driving virtually any cans you throw at it, from 16 to 600 ohms. It'll be way better than the old fashioned series resistor, which works very poorly with modern low impedance headphones.
Thanks. Yep, been looking at headphone amps and there is a build sometime in my future.
I did initially think that I could challenge myself to make one using only the parts in this amp. BUt if I am building a headphone amp I want it to be a good one. The idea with this is simply to use it as a way to learn my way around a circuit diagram and how this translates to a pcb, also to practice some de/soldering and other techniques.
Having fun so that is the important bit.
"Old phono pre amp as headphone buffer? "
not a good idea, for two reasons, as all phono preamps are only meant to provide line level signal with little driving capability, that is not to drive low impedance loads, and second, more importantly, they do not have flat fr response, but RIAA shaped, to accommodate opposite fr response applied to LP cutting. This RIAA is in cheap preamps applied in feedback, in order to make it flat, you could replace it with just a resistor, but the circuit may be instable, as phono preamps are designed to have much higher gain as just a buffer...and in the end it will sound like crap.
Listen to Fast Eddie D...there is a ton of good circuits.
not a good idea, for two reasons, as all phono preamps are only meant to provide line level signal with little driving capability, that is not to drive low impedance loads, and second, more importantly, they do not have flat fr response, but RIAA shaped, to accommodate opposite fr response applied to LP cutting. This RIAA is in cheap preamps applied in feedback, in order to make it flat, you could replace it with just a resistor, but the circuit may be instable, as phono preamps are designed to have much higher gain as just a buffer...and in the end it will sound like crap.
Listen to Fast Eddie D...there is a ton of good circuits.
Here's a new generation op amp that would probably make a good headphone amplifier without an output buffer. Linear - Amplifiers - Instrumentation, OP Amps, Buffer Amps | Integrated Circuits (ICs) | DigiKey It's SMD and expensive though.
You could build a nice headphone amp with OPA2134 + 2 BUF634s using readily available, through hole parts. BUF634s are available in DIP-8 and TO-220-5 packages. It's maybe not the cheapest, but it's simple and can be made very compact. It can be built on a breadboard. I keep a stack of these https://www.radioshack.com/collecti...iversal-component-pc-board?variant=5717555141 on hand. They're perfect for any op amp based prototype circuit. I've been using some of my "prototypes" built on these boards for years.
Edit: I build a NE5532/OPA2134 + 2xBUF634 single ended headphone amp on that Radio Shack protoboard, complete with all capacitors, that I cut down to 2 1/2" by 2 3/4". You could make it even smaller without the bulky (4x500 uF 50 volt) output capacitors. Single ended has more signal capacitors and a bunch of anti latch up/ anti thump diodes, too; plus an active bias circuit and slow start. All on a 2 1/2" + 2 3/4" board, plug and play. I buy super tiny (and expen$ive) CMF50 resistors, small package electrolytic capacitors where I can get away with it. I use DIP-8 package BUF634s. It's all through hole parts.
You could build a nice headphone amp with OPA2134 + 2 BUF634s using readily available, through hole parts. BUF634s are available in DIP-8 and TO-220-5 packages. It's maybe not the cheapest, but it's simple and can be made very compact. It can be built on a breadboard. I keep a stack of these https://www.radioshack.com/collecti...iversal-component-pc-board?variant=5717555141 on hand. They're perfect for any op amp based prototype circuit. I've been using some of my "prototypes" built on these boards for years.
Edit: I build a NE5532/OPA2134 + 2xBUF634 single ended headphone amp on that Radio Shack protoboard, complete with all capacitors, that I cut down to 2 1/2" by 2 3/4". You could make it even smaller without the bulky (4x500 uF 50 volt) output capacitors. Single ended has more signal capacitors and a bunch of anti latch up/ anti thump diodes, too; plus an active bias circuit and slow start. All on a 2 1/2" + 2 3/4" board, plug and play. I buy super tiny (and expen$ive) CMF50 resistors, small package electrolytic capacitors where I can get away with it. I use DIP-8 package BUF634s. It's all through hole parts.
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Had a look at the schematic of this amp. One of the finest pieces of '80s BPC I ever saw.
I'm afraid the existing components won't be cutting it. The phono preamp supply is a dropper resistor + zener job that'll supply under 25 mA tops, and a '4558 is among the wimpiest audio opamps you'll find (not much better than a TL07x).
That being said, you could turn the phono stage into a fairly serviceable headphone amp if you were to drop in an NJM4556AD instead and invest into a few resistors in the 1k2-2k2 vicinity to replace R409/410 (plus maybe 10 ohms to replace R415/416 for output series resistors to be safe - this opamp can do without given a good layout, but I wouldn't be expecting one). Remove C409-412 and R403/404, short R411/412, replace C403/404 by C415/416. Some extra local rail decoupling (at least between rails) and beefing up C807/808 to more like 470-1000 µF each is also recommended. Should do an alright job, just don't expect any real miracles attempting to drive power-hungry low-impedance cans at high volume, as the supplies would surely collapse. It would certainly be beneficial for '4556A dissipation if they were to give a bit under load, but not as much as this.
I'm afraid the existing components won't be cutting it. The phono preamp supply is a dropper resistor + zener job that'll supply under 25 mA tops, and a '4558 is among the wimpiest audio opamps you'll find (not much better than a TL07x).
That being said, you could turn the phono stage into a fairly serviceable headphone amp if you were to drop in an NJM4556AD instead and invest into a few resistors in the 1k2-2k2 vicinity to replace R409/410 (plus maybe 10 ohms to replace R415/416 for output series resistors to be safe - this opamp can do without given a good layout, but I wouldn't be expecting one). Remove C409-412 and R403/404, short R411/412, replace C403/404 by C415/416. Some extra local rail decoupling (at least between rails) and beefing up C807/808 to more like 470-1000 µF each is also recommended. Should do an alright job, just don't expect any real miracles attempting to drive power-hungry low-impedance cans at high volume, as the supplies would surely collapse. It would certainly be beneficial for '4556A dissipation if they were to give a bit under load, but not as much as this.
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That sure leaves a lot to be desired. For starters, it has a 100 ohm output impedance.
That'[s Doug Self's circuit and I'm sure it would work better. I bet you can't fit it on a 2.5" square proto board from Radio Shack though.
Had to look that one up >> BPC
I was always suspicious of it as it is less than 20cm deep - certainly could stack much on top of it!
Adason - thanks for the comprehensive answer. I can see that's not going to work as planned. From a completely ignorant understanding I always thought a phono pre-amp was to boost the signal somehow hence why I thought it would be a good substitute headphone buffer.
Fast Eddie, adason, and others > all good stuff. I'll come back to this as a summary when I do make a proper headphone amp. Had been looking at the O2 amp but can see there are plenty of other varieties to choose from.
This discussion has made me think about what I would will be doing with this project beyond the personal challenge I have set myself. I'll probably drop the headphone output for now - I was planning to pass this on to a friend to use in her art studio (along with some small Kefs I have in the cupboard) so probably not much headphone use and besides I was having problems as to where I would fit the socket. But if I like the casing I end up with maybe I'll strip it all out again and try one of the schematics from this thread and make that headphone amp that I know I should make.
Cheers. GOod night
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