Modified Paramour 2 (SR-45) as headphone amp?

[Mikey]

Dan, I'm sorry if this post is in the wrong sub-forum...please feel free to move it if necessary!

Looking at the schematics for the Paramour 2 and the SEX amp side by side, there are many similarities.
Both have an input stage capacitor coupled to a parafeed output stage.
Both are capable of driving loudspeakers.
The SEX amp can drive headphones, so couldn't the Paramours be wired to drive headphones too?

I'm going to give it a shot this weekend and see what happens, but I'm curious what I'd have to do to optimize this setup should it turn out to hold some promise...

Mike

[Paul Joppa]

Paramour is a DHT with AC filament power. Therefor it has some hum even when optimally adjusted. Sensitive headphones will have unacceptable hum for this reason, though some of the less sensitive ones might work just fine. Notice that the SEX amp has DC heater power even though the heater power is isolated by the separate cathode from most of the hum problem. We had to add the DC heater in order to get the hum down to acceptable for a headphone amp. We don't add cost to our most affordable amp casually!

[Mikey]

Paul, as you suspected, there was a bit too much hum for low-level headphone listening.
However, when the volume was turned up enought to mask the hum, the sound was very good!

I might try converting to DC heaters some time in the future and see what happens....

Mike

[Mikey]

Paul, which would have the greater effect on reducing the hum:
Converting to DC heaters on the output tubes or the driver tubes?

Would this kit (without transformers) be a suitable choice?

http://welbornelabs.com/ps6.htm

Mike

[ironbut]

Hey Mikey,
Not that I know anything about anything,.. you might try using high frequency AC.
I've fooled around with 211's using modified lighting transformers for the heaters. They work okay regarding hum, but those lighting transformers produce some ugly squarish waves. One of these days I'm gonna figure out how to build some 60KHz+ sine wave heater supplies.

[Doc B.]

Start with the filament supply for the 45s.

[Mikey]

I built a pair of filament supply boards tonight, but need some guidance installing them into my amps.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi3.photobucket.com%2Falbums%2Fy80%2Fmpaschetto%2FParamour%25202%2520Amps%2FDSC09012_zps59e4aa9b.jpg&hash=ba8a448f3f5368acbecc36d5e62f4c80d19b8191)

I'm guessing that I should simply disconnect the red and black wiresfrom pins 1 & 4 of the tube socket, and splice the boards in.

 
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi3.photobucket.com%2Falbums%2Fy80%2Fmpaschetto%2FParamour%25202%2520Amps%2FDSC09014_zps966abf9b.jpg&hash=b9892caea83f5fdf70f970d7581d7667c82118e7)

Is it that simple, or is there more to it than that?

Mike

[Paul Joppa]

I though I had replied some time ago, but I don't see my post currently so it's either been lost or my posting failed.

It is something between difficult and impossible to get 2.5vDC from a 2.5vAC supply - the diode voltage drop is too high. That's why I have been struggling (off and on!) for a few years to make a sensible SR45 with DC filaments. You can try 8-amp Schottkys with a single large capacitor - there are some 39000uF caps at Mouser. I worked this up for the Stereomour but nobody was interested enough to try it and I don't have one myself, so I don't know whether it will generate enough voltage or not.

You can't use a FW bridge (what appears to be in your photo) with the 6.3v winding because it cannot handle the current. You can't even use a FWCT rectifier off the 6.3vCT winding, since it also has to power the driver/shunt reg tube. If the 2.5v winding/single cap does not make enough voltage, or does but the ripple is too great, then you'll have to get another power transformer for the filament power. If you go for a new power transformer, consider a regulator such as the LT1963 series which are relatively quiet and available in a 2.5v version. Just a thought.

[Mikey]

Hi Paul,

Yep, that is a full wave bridge consisting of four 1N5822 diodes.
It is followed by a C-R-C filter at the moment.

According to the research I did yesterday, a capacitor input power supply with a full wave
bridge will produce 1.414 times the AC voltage presented to it (maximum).  In this case,
that would be 3.54 VDC.  Subtracting the diode voltage drop (.525 V), I'm still coming up
with around 3.0 VDC nominally.

I figured I'd need the resistor in there (0.5 ohm) to drop a bit of voltage...what am I missing?


If I need to reconfigure the board to be a single large capacitor after the bridge, I can certainly
remove the resistor and place those two caps in parallel....and then give it a try!

Mike

[braubeat]

You must subtract the forward voltage drop before you do the capacitor input computation. So you have (max) 2 volts presented to the cap. 2 volts times 1.4(max) = 2.8 volts. This of course is more than you ever actually get.

Michael

[Mikey]

You must subtract the forward voltage drop before you do the capacitor input computation.

D'oh!  There's the eureka moment!
I get it now...

I'll remove the resistor, parallel the caps, and splice the board into the circuit.
Then we'll see what voltage I measure once it's installed.

[Paul Birkeland]

Hello Mike,

Yes, what you have laid out will often times present a mostly-accurate model that can be used to calculate power supply voltages, except for those cases of low voltage and high current.  Notice that when we use these to calculate the output voltage of a 300V winding, we are not too concerned if the model is off by a couple of volts.

One factor that will lower your voltage and leave you a bit stumped is the voltage regulation of the power transformer.  As you change from AC to DC, you begin to tax the power transformer a bit more, and your 2.5A load will be seen by the PT as something more like 3.25A, and consequently the AC output voltage will sag a bit.  I don't know what this drop will be in your particular situation, but it will certainly present itself.  

With the bridge rectifier, you will have two of those diodes conducting at all times, and you'll probably lose around 1V based on your load current, which is a huge percentage of the total available voltage.  

With those two factors alone, the "Available" AC voltage may be more like 1.4V, and even with just the bridge and one capacitor, you may not be able to get it back up to 2.5V DC.

On the other hand, you can tuck a 5V/4A power transformer under the hood, bridge rectify that, then select an appropriate dropping resistor (or regulator) based on your results (that's whay I'd do).

-PB

[Grainger49]

Mike,

Beautiful work!  It has a lot of space.  I ran out of space in my Paramours (original) years ago.

[Mikey]

On the other hand, you can tuck a 5V/4A power transformer under the hood, bridge rectify that, then select an appropriate dropping resistor (or regulator) based on your results (that's whay I'd do).

-PB

Hi PB,

Thanks for the input, it will save me from wasting some time.
I'll eventually pick up a pair of appropriate transformers and do this the right way...

Mike

[Paul Birkeland]

In the mean time, did you try adding some series resistance between the speaker outputs and your headphone jack?

You may find that this will diminish the noise to an acceptable level for your uses.

-PB