PANDA Headphone amp kit, build thread, upgrades & tweaks.

[Deleted]

Hi Syd,

There is an 18V zener diode near the transistor with the 6.1.
Measure the voltage across it.
I expect around 13.8V over it.

This could explain the hum as the voltage regulator does not regulate anymore.

Replace the transistor in this case and it should work O.K. again.
Also consider the PS modifications (proper calculation of the zener diode resistor) when you are at it.

[Deleted]

Apr 2, 2011 12:07:55 GMT @solderdude said:

Hi Syd,

There is an 18V zener diode near the transistor with the 6.1.
Measure the voltage across it. I expect around 13.8V over it.

This could explain the hum as the voltage regulator does not regulate anymore. Replace the transistor in this case and it should work O.K. again.

Also consider the PS modifications (proper calculation of the zener diode resistor) when you are at it.

Frans

Voltage is as you say. I'll order up a new transistor. Should I replace the zener?

I've read up the PS resistor adjustment and I'll carry that mod out.

Thanks for all your help Frans.

Syd

[Deleted]

Hi Frans

The 669 transistor finally arrived today. I fitted it and the zener voltage is now 17.9. The 669 reads very similar to the one in the other channel.

Unfortunately the hum is still there and quite loud. I measured the output at the connector block on the board. One channel is 11mv, the other 22ish. Is there anything else I could try?

Thanks, Syd

[Deleted]

What is the voltage on the replaced transistor (Base Collector & Emitter measured opposte ground)

What is the voltage across the 3V9 zener in both amps ?

[Deleted]

Apr 19, 2011 14:10:41 GMT @solderdude said:

What is the voltage on the replaced transistor (Base Collector & Emitter measured opposte ground)

What is the voltage across the 3V9 zener in both amps ?

B 17.9, C 25, E 17.2 Both zeners 3.71V

[Deleted]

hmmm that appears to be O.K.

the hum is only in the channel that had the faulty transistor ?

[Deleted]

The faulty transistor was on the side of the board that has the phono inputs on. I couldn't quite work out which side that feeds!

Please don't waste too much time on this, your help so far has been great.

[PinkFloyd]

I reckon the ground plane has been disturbed somewhere.... quite easy to "break" a through hole connection with too much heat and a BASTARD to find..... get a good ground reference point and do a continuity check from EVERY ground joint that should connect to it.... I think you will find the culprit .... the one joint that doesn't register an audible buzz (on your multimeter's continuity "audible" setting) is the bugger that's causing the hum..... point to point a wire from that "part" to ground and your hum will disappear.... As I say, a BITCH to pinpoint and the only way you will find it is with "time"

[dicky]

Hi folks,

for some inexplicable reason, my Panda has started blowing fuses.

She's been running fine since Christmas and has been fitted with 250mA slow-blow fuses since she was born.

Last night she blew 4 fuses in succession on switch-on. I tried a 500mA F and that blew. She's currently running on a 1A F.

Does anyone have any idea why this is happening and, more importantly, is a 1A F OK for long term use?

Regards,

Dicky

[Deleted]

Is the trafo getting hotter than before ?

Are the heatsinks in both channels equally hot ?

If the fuse is in the primary I wouldn't use F (fast blow) types but only Slow or Antisurge types or Time-lag.

Fuses generally blow on start-up because of high inrush currents.
If you have a 60VA trafo 0.25A might be a bit 'tight'.
This might work O.K. for quite some time but every time you switch it on the fuse get's a jolt till it finally fails.

using 0.5A Fast will blow immediatly as the inrush current is higher than 0.5A
Your 1A fuse might handle the inrush current but might not fail when needed.

I recommend 0.315A/T or perhaps even 0.4A/T in case of a 60VA transformer.

[dicky]

Hi Frans,

No, the trafo is not hotter than before - oh, and its 80VA.

All heatsinks are the same temperature (hot - according to Mk1 thumb - but no hotter than when she was new).

The fuse is in the primary - so I'll go for 0.5A/T?

Is there a rule-of-thumb?

Cheers,

Dicky

[Deleted]

I would go for 0.4A/T

This will blow when the transformer is just above it's maximum power rating and is high enough to withstand the startup currents.
When there is a short circuit the fuse will blow (that's what it is essentially there for).

80VA is a bit overkill ... as max 12W will be drawn (to get 2 x 0.2 W output power)
a 30W would have been more than acceptable already.

It seems to me the amp and trafo are still in perfect working order and the fuse is likely to have blown because of the repeated high inrush currents.

[dicky]

Thanks Frans,

0.4A/T it is then.

I'm using the 80VA because it was lying around - should I change it?

[Deleted]

Nah .. a bit overdimensioning things hasn't hurt anyone (except a few fuses ? :

Just pop in the 0.4A/T fuse and see if this 'holds'...

[dicky]

Thanks, Frans. It's odd that it's just started blowing them.

I haven't used it for a while - I've been using my V2.

Could something have aged or run-in to cause a change? Why would the in-rush current be higher? As I saud before, it's been fine using 0.25/T for 7 months!

[pandapops]

Two somewhat techie questions for this thread:

1. That 22pf ceramic DC blocking cap is a very odd design feature, normally you'd expect a value much greater 0.1uf-10uf, surely this can improved upon, any suggestions for better cap or even a line transformer?
A little reading suggests that a higher capacitence value brings lower impedance and a lower resonant frequency, but the resonant frequency is operating with a huge headroom given the 22pf cap.

2. What sort of gains would be had if each channel had its own power transformer( + rectifier)? This can easily be done with those 2 ohm resistors providing a break to the power rail. How about output stage and main section with a transformer each?

[Deleted]

1: the 22pF cap is not a coupling cap but a 'miller' cap in the overall feedback.
It prevents oscillation.
Increasing the value limits the frequency response of the amplifier.
I do not recommend to make it smaller without the aid of an oscilloscope that can do at least 10MHz.
a DC blocking cap (so DC gain would become only 1x) would have been in series with R13 and R29 and have to be at least 220uF.
but the best cap in an audiopath is no cap.
The Panda does not have any coupling caps... it is DC coupled.

a blocking cap on the input (between wiper and 1MOhm resistor) should be at least 100nF but would increase noise.
a blocking cap at the input of the amp (before the volpot) would have to be several uF minimum (depending on the resistance of the volpot)

2: the differences would be purely subjective so they might be 'huge' to some and 'indescernable' to others.
The PCB layout is not designed for dual power supply operation (in case you want more of a dual mono construction).

Since it is a full class-A design there is little reason to decouple the pre and output stage power supply as well as left and right channel.
They simply can't influence each other as there is no 'audio type' of current drawn which would validate such an attempt.
It could be beneficial for class AB amps with poor PSSR.
Simply replacing a resistor in the pre-part PS section with a CCS (FET with gate and drain connected) already isolates enough IMO (for hum introduced artifacts in the pre-stage).
Leo suggested this early on in the thread.
You need to find a FET with the proper specs to get it right.
Also in this case the audio improvements are highly subjective.
If subjectivity is your goal making a dual power supply will almost certain increase the soundquality.

[pandapops]

Thanks Solderdude, I had literally never heard of a Miller cap before.

Is there any point in a regulated PSU, which would give the output stage a cleaner supply and the first stage regulator less crap to deal with?

Just looking for any simple improvements that a solder by numbers person could do.

How about tried those Toshiba transistors from the JLH ripple boards instead of the TIP41/42s?

[Deleted]

I have listened to one for hum, fed with a linear (lab) power supply and by using a single 15VA transformer.
I couldn't tell the difference (hum wise) so the regulator section appears to be O.K.
I could not hear any hum only a slight hisssss with a sensitive headphone and NO output resistors.
So you can feed it from a regulated supply but doubt it will enhance things.
Perhaps it will in subjective area's though.

The amp itself is limited to about 3MHz bandwidth.
The output transistors themselves (TIP41/42) are also 3MHz, (2SC5171 is 200MHz).
When you put in the fastest transistors in the world the bandwidth (which is limited by the 22pF cap) will remain the same.
Only if you have a scope you can safely extend the FR by using faster transistors AND changing (various) miller caps in the circuit depending on the newly formed phase characteristics.

The question is... is 3000 KHz bandwidth not enough ?
With the 2SC5171 and other caps you might reach 10000 kHz.
Even on the highest of hires recordings with the best of the best headphones I would be surprised if you could reach (usefull) sound related info upto 40 kHz so the amp is already 75x faster than that IF it were there and could be reproduced even at maximum output power.
Me, only hearing upto 16 kHz, the Panda's bandwidth already exceeds my hearing by almost a factor 200 !

In case you believe they 'add' a sound signature, replacing the transistors can be a good idea.
You might need to check for oscillations...

another question is do you really want amplification well within the videoband ?
The 2SC5171 e.t.c. were intended for CRT video cathode stages.
I firmly believe a bandwidth of 100kHz (-0.5dB) is already heavy overkill, would be about 300kHz (-3dB) and see no real point in bandwidths of 1MHz and higher.
You will only risk the chance of amplifying garbage you really wouldn't want amplified.

[pandapops]

Is this amp pretty much unupgradeable without significant re-design then?
The only thing I can think of is a fancy volume control.

Specifically I'd like better low frequency definition, I had the same issue with another amp using, I think TIP31/32, so was hoping these TIP things were the issue.

[joethearachnid]

Lowering the output impedance value can tighten the bass somewhat. I have a socket fitted to the back of my Panda with zero output impedance for orthos and other hard-to-drive cans, and through that bass definitely feels tighter but not as impactful. Treble also seems a bit less smooth. A halfway point from the 120ohm standard like 40/50ohms might help.

-JoetheArachnid

[Deleted]

Joe is absolutely correct.

It's not the output transistors but the combination of max output voltage, max output current, output resistance, headphone impedance and maximum power rating of the headphone in question.

With some headphones having a high output impedance is not the most ideal situation and is most 'active' in the lows and highs area.
lowering the output impedance helps but in case of the Panda, where no current limiting is applied, you must be carefull using the low output impedance with low Ohmic headphones.
In an unguarded moment a sudden burst of high volume can simply burn out low-Ohmic drivers.
If you only use high Ohmic drivers you don't have anything to worry about though and can safely use low (to even no) output resistors.
Some headphones are simply designed to give optimal performance with 120 Ohm output resistance.
Other headphones (mostly low-Ohmic or portables) are designed with low output resistances in mind (but also low maximum voltages)
The maximum voltage of a Panda could destroy these headphones if a resistor is not fitted.

[pandapops]

Using HD600 with the amp in burn-in lash-up setup with no resistors on output. Finding the highs a little artificial, but much better than IC based amps tend to be, and lows just difficult to tell what the source of some bass is.

[Deleted]

The HD600 can be safely connected to the Panda without any resistors in series.
Just try how you like it with 0 Ohm, 120 Ohm and perhaps something in between.
There is bound to be a 'sweet spot' for you and the HD600.

[pandapops]

Okay, thanks.

Did you knoew hifidiy.net had an older amp kit:
cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=120687981479&ssPageName=STRK:MEWAX:IT

Its rather similar to the NVA amp which the Panda sounds similar to.