I’m also using a JJ ECC83S on the input, for maximum “bite”. This tube is closer to the E803CC of old…
Any opinions / reviews on Gemtune APPJ PA1502A
- Thread starter Mortus
- Start date
I am late to the party.
I recently got a used PA1502A, and took it apart for some cleaning and refurbishing. The build quality is very good, and APPJ don't cut corner in the design phase, everything seems to be well thought out, there is a large heatsink for the power supply control chip (TOP250YN), a large sheet metal shield over the switching power supply section, there is even a large silicon thermal pad to transfer the heat from the PCB to the metal shield. Comparing to my Little Bear P8, which is at about the same price, this APPJ is super well made. (Little Bear P8 ==> so so build quality, sub-standard design)
Tube rolling so far (configuration = 0.44uf FT3 as interstage coupling caps; volume control changed to shunt-pot type, with 22k AudioNote as input resistor; 6F6 as output tubes; gain switch=H):
I have tried 12ax7 and 12au7, both are amperax bulgleboy measured as "new". I much prefer the 12ax7, much cleaner and fuller sound.
with 12au7 installed there is more hum, and the sound is "harder", not as rich. The plate voltage is at just 55v (125v with 12ax7 installed). To properly use the 12au7, I think the cathode resistor value needs to be adjusted to get proper plate voltage. I am happy with the 12ax7 so I am not gonna mess with that.
coupling capacitors: so far I have tried FT3 (aluminum/teflon) and K40Y9 (paper in oil). The K40Y9 has a more "simplified" sound but is smoother and more easy-listening. I will try some other caps later (Dynamic, Vitamin-Q, AudioCap Theta etc).
(minor) complains about the PA1502A:
-- volume pot is tiny, there is obvious channel imbalance at low volume. This is not a real problem with my current phones (Forstex T50rp), the full listening volume happens with volume pot at around 12 o'clock anyway.
-- there is a low hum, audiable when music is not playing. The filament supply is DC so I was surprised to hear this hum. It doesn't sound like the typical 60hz. I will investigate this further.
I recently got a used PA1502A, and took it apart for some cleaning and refurbishing. The build quality is very good, and APPJ don't cut corner in the design phase, everything seems to be well thought out, there is a large heatsink for the power supply control chip (TOP250YN), a large sheet metal shield over the switching power supply section, there is even a large silicon thermal pad to transfer the heat from the PCB to the metal shield. Comparing to my Little Bear P8, which is at about the same price, this APPJ is super well made. (Little Bear P8 ==> so so build quality, sub-standard design)
Tube rolling so far (configuration = 0.44uf FT3 as interstage coupling caps; volume control changed to shunt-pot type, with 22k AudioNote as input resistor; 6F6 as output tubes; gain switch=H):
I have tried 12ax7 and 12au7, both are amperax bulgleboy measured as "new". I much prefer the 12ax7, much cleaner and fuller sound.
with 12au7 installed there is more hum, and the sound is "harder", not as rich. The plate voltage is at just 55v (125v with 12ax7 installed). To properly use the 12au7, I think the cathode resistor value needs to be adjusted to get proper plate voltage. I am happy with the 12ax7 so I am not gonna mess with that.
coupling capacitors: so far I have tried FT3 (aluminum/teflon) and K40Y9 (paper in oil). The K40Y9 has a more "simplified" sound but is smoother and more easy-listening. I will try some other caps later (Dynamic, Vitamin-Q, AudioCap Theta etc).
(minor) complains about the PA1502A:
-- volume pot is tiny, there is obvious channel imbalance at low volume. This is not a real problem with my current phones (Forstex T50rp), the full listening volume happens with volume pot at around 12 o'clock anyway.
-- there is a low hum, audiable when music is not playing. The filament supply is DC so I was surprised to hear this hum. It doesn't sound like the typical 60hz. I will investigate this further.
thank you very much for your great work, much appreciated. It makes tinkering with this little amp much more fun.
back ground noise / hum:
the hum is audiable with my T50rp, but it is very obvious with my 32 ohm Beyer 770. The waveform taken at the phone jack is not what I have expected,
(measured with T50rp plugged into the jack)
peak-to-peak amplitude is ~500mV, the space between spikes is about 16uS, so ~65kHz. The power supply IC, TOP250YN runs at 132kHz, these spikes are likely from the switcher. it looks like the noise from the switching power supply has gotten into the signal path. The "hum" I am hearing might be some kind of beat frequency.
The HV rail looks relativly clean but does have these spikes. Scope capture shows 1.2v peak-to-peak in AC-couple mode, but 8Vp-p in DC-couple mode.
Filament supply has these spikes too, ~1.5Vp-p
Filament supply circuit: D3 is the rectifier (gets hot when running the amp), C04 (1500uf/16v) is the first filtering cap, current goes through L04 (3.3uH) to the 2nd filtering cap C05 (1500uf/16v). I changed L04 to 700uH, the spikes went down to 0.7Vp-p. The hum is a little quieter but still annoying when using 32 ohm Beyerdynamic.
increasing C05 capacitance did not make much improvement either.
I will try using a separate supply to provide the filament voltage, maybe that can help getting rid of the hum. Not sure if this will affect the HV rail though, (the feedback for TOP250YN is taken from the 12v rail before L04).
update:
I could not make the spikes go way with what I have in my parts box. The largest coil I have tried is 1.5mH and the spikes at the output jack went down to around 300mVpp, the hum was still there and sounded more or less the same.
At this point I am convinced that while the spikes are real, it is outside of my hearing range, the hum I am hearing is probably something picked up by the long lead wires I am using in the signal-cap test rig.
update:
I connected a DC supply to provide the 12v filament voltage, now the switching supply on the board only has to provide HV voltage ==> 174v DC, with 6Vpp spikes; power consumption of the amp is 11w/22va; (correction but hum is still there.
Different HV reservoir caps have noticeable effect in this amp, I tried ASC polyprop vs Nichicon KZ electrolytics (three 100v KZ in series), the KZ yields more solid impact while the sound with polyprop is more spacious with richer bass details. I much prefer the polyprop.
the hum is audiable with my T50rp, but it is very obvious with my 32 ohm Beyer 770. The waveform taken at the phone jack is not what I have expected,
(measured with T50rp plugged into the jack)
peak-to-peak amplitude is ~500mV, the space between spikes is about 16uS, so ~65kHz. The power supply IC, TOP250YN runs at 132kHz, these spikes are likely from the switcher. it looks like the noise from the switching power supply has gotten into the signal path. The "hum" I am hearing might be some kind of beat frequency.
The HV rail looks relativly clean but does have these spikes. Scope capture shows 1.2v peak-to-peak in AC-couple mode, but 8Vp-p in DC-couple mode.
Filament supply has these spikes too, ~1.5Vp-p
Filament supply circuit: D3 is the rectifier (gets hot when running the amp), C04 (1500uf/16v) is the first filtering cap, current goes through L04 (3.3uH) to the 2nd filtering cap C05 (1500uf/16v). I changed L04 to 700uH, the spikes went down to 0.7Vp-p. The hum is a little quieter but still annoying when using 32 ohm Beyerdynamic.
increasing C05 capacitance did not make much improvement either.
I will try using a separate supply to provide the filament voltage, maybe that can help getting rid of the hum. Not sure if this will affect the HV rail though, (the feedback for TOP250YN is taken from the 12v rail before L04).
update:
I could not make the spikes go way with what I have in my parts box. The largest coil I have tried is 1.5mH and the spikes at the output jack went down to around 300mVpp, the hum was still there and sounded more or less the same.
At this point I am convinced that while the spikes are real, it is outside of my hearing range, the hum I am hearing is probably something picked up by the long lead wires I am using in the signal-cap test rig.
update:
I connected a DC supply to provide the 12v filament voltage, now the switching supply on the board only has to provide HV voltage ==> 174v DC, with 6Vpp spikes; power consumption of the amp is 11w/22va; (correction
but hum is still there. Different HV reservoir caps have noticeable effect in this amp, I tried ASC polyprop vs Nichicon KZ electrolytics (three 100v KZ in series), the KZ yields more solid impact while the sound with polyprop is more spacious with richer bass details. I much prefer the polyprop.
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baronbeehive
Headphoneus Supremus
Hi AudioCats, just noticed a new member here after all this time!
Just reply to a couple of points, nobody else has mentioned a hum so I wonder if it is something to do with your amp or connections? As you say the SMPS does exhibit noise but usually outside the range of human hearing unless your hearing or headphone is ultra sensitive. And the metal shield should do the job of stopping PSU noise entering the audio signal. I must admit I was initially concerned about the SMPS after hearing horror stories about them spontaneously combusting but after many years of use, and some modding, it has remained good.. touch wood!
Re: cathode bypass caps I have the Nichicon KZ's in my other amp and love them but for the APPJ I have the Audio Note Kaisei's which do an excellent job of rounding out the sound to a pleasing weight and resolution. What capacitance have you installed here? I'm not really qualified to comment on the headphone APPJ you have as mine is the speaker version but with a low power output the higher the better for power handling, mine are 1000uF with excellent results.
You mention 12Au7, I recently tried a 12AT7 Brimar from Footscray factory, and I was very surprised when it turned out to better my Philips Herleen factory tube because it is much lower gain, so it is now permanently installed in the amp. The SQ from the amp now matches my other highly modded amp which is a Little Dot MKVI+. Not bad for a lowly $150 amp!
Just reply to a couple of points, nobody else has mentioned a hum so I wonder if it is something to do with your amp or connections? As you say the SMPS does exhibit noise but usually outside the range of human hearing unless your hearing or headphone is ultra sensitive. And the metal shield should do the job of stopping PSU noise entering the audio signal. I must admit I was initially concerned about the SMPS after hearing horror stories about them spontaneously combusting but after many years of use, and some modding, it has remained good.. touch wood!
Re: cathode bypass caps I have the Nichicon KZ's in my other amp and love them but for the APPJ I have the Audio Note Kaisei's which do an excellent job of rounding out the sound to a pleasing weight and resolution. What capacitance have you installed here? I'm not really qualified to comment on the headphone APPJ you have as mine is the speaker version but with a low power output the higher the better for power handling, mine are 1000uF with excellent results.
You mention 12Au7, I recently tried a 12AT7 Brimar from Footscray factory, and I was very surprised when it turned out to better my Philips Herleen factory tube because it is much lower gain, so it is now permanently installed in the amp. The SQ from the amp now matches my other highly modded amp which is a Little Dot MKVI+. Not bad for a lowly $150 amp!
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yeah, the hum I am hearing is likely (mostly) from my capacitor test rig (long wires connecting to header blocks, allowing fast coupling capacitor change without soldering).
I haven't tried different cathode bypass caps yet. The voltage at 6v6 cathode measured around 12v, I will try some 1000uf/25v BG-std and see what happens.
I haven't tried different cathode bypass caps yet. The voltage at 6v6 cathode measured around 12v, I will try some 1000uf/25v BG-std and see what happens.
baronbeehive
Headphoneus Supremus
If you do try those 1000uF's you will need to parallel some smaller bypasses to help with the higher frequencies. Without them the high capacitance will help with power handling but will adversely affect the sound producing a rather heavy sound.
There is a very handy calculator for finding the best cathode bypass capacitance here: https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/
It may be that 1000uF is overkill if you look at the calculator but it should certainly benefit power handling anyway and has no adverse effects such as oscillations.
There is a very handy calculator for finding the best cathode bypass capacitance here: https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/
It may be that 1000uF is overkill if you look at the calculator but it should certainly benefit power handling anyway and has no adverse effects such as oscillations.
I think appj people put a lot of thoughts into the design, I don't worry about heat problem in this little amp. It was not super warm when I was running it in full stock condition, though only for about an hour.
I did measure power supply component temps before mods. The hottest part was D3 (rectifier diode for filament supply, doing about 1A), it reached 65C after a few hours (with the amp rested on the side with bottom plate removed). I suppose it can get to 80~85c during normal operation. C04 is next to it so can see some rather warm temperature, and APPJ chose a 1500uf/16v Rubycon that is rated to 130C (!). I was impressed when I saw that.
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baronbeehive
Headphoneus Supremus
Right! Good information, I didn't get round to looking at the schematic, so didn't know that about the 130C component, I'm pretty impressed with that too. I did make my own rudimentary schematic with the connections for the purposes of the alterations.
You're right the amp doesn't run hot so I've never had any worries.
Partial schematic of PA1502a's power supply. I didn't bother to trace anything related to the power control IC (TOP250YN) since I don't plan to make any change there.
Q1 (4N60, 4A/600v rated) and C06 form a capacitance multiplier.
The TOP chip monitors the filament line voltage, IC2 is the opto-isolator to relay feedback back to the TOP chip. HV/B+ line is unregulated.
Q1 (4N60, 4A/600v rated) and C06 form a capacitance multiplier.
The TOP chip monitors the filament line voltage, IC2 is the opto-isolator to relay feedback back to the TOP chip. HV/B+ line is unregulated.
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result of my 6v6 cathode caps experiments:
(shunt-pot input resistor=audioNote; interstage coupling=FT3 teflon; phones= 32ohm Beyer)
a) ASC 20uf polyprop: boomier bass than stock, hollower sounding; has low frequency "motor boating" oscillation.
b) 1000uf/25v BG-std: solid bass, with rich bass details. a bit cleaner sound than stock. Bass impact might be a little too hard/overwhelming.
c) Nichicon ES (green) bipolar 220uf/50v, long lead to + side: bass between BG-std and stock; cleaner sound than stock; overly warm, a bit rounded-off?
stock cap has softer boomier bass than BG-std, more lay back, duller sound with less focus.
The differences are not night and day. Nichicon ES is good enough, and it is the same size as stock, fits perfectly.
(I wanted to try Kaisei but the values I want are out of stock at partsconnexion.)
(shunt-pot input resistor=audioNote; interstage coupling=FT3 teflon; phones= 32ohm Beyer)
a) ASC 20uf polyprop: boomier bass than stock, hollower sounding; has low frequency "motor boating" oscillation.
b) 1000uf/25v BG-std: solid bass, with rich bass details. a bit cleaner sound than stock. Bass impact might be a little too hard/overwhelming.
c) Nichicon ES (green) bipolar 220uf/50v, long lead to + side: bass between BG-std and stock; cleaner sound than stock; overly warm, a bit rounded-off?
stock cap has softer boomier bass than BG-std, more lay back, duller sound with less focus.
The differences are not night and day. Nichicon ES is good enough, and it is the same size as stock, fits perfectly.
(I wanted to try Kaisei but the values I want are out of stock at partsconnexion.)
baronbeehive
Headphoneus Supremus
Yes, this amp as stock is very good as is but is really needing some basic mods such as we are doing here to bring out the best, which is endgame AFAIK.
That cathode bypass calculator I linked to is very interesting, I haven't tried it with this amp because I didn't know some of the inputs. But, for example the frequency curve for a 1000uF cap shows suspected overkill but, as there is no oscillation in the APPJ, I have stuck with it because of the excellent sound and power handling. In the cathode bypass calculator example above there was no difference in the frequency curve for a 500uF or 1000uF cap hence suspected overkill.
I tried initially with 1000uF in this amp and found as suspected that the sound was heavy, dull and the higher frequency sparkle were lost. As soon as I installed the smaller Audyn True Copper caps as bypasses the sound became very lively again. Without that I suspect that something around 500uF might suffice on its own.
I haven't found the bass overpowering or boomy at all on the contrary it is sharp, detailed and in proportion to the rest of the frequency band. The treble also is extended and I find the whole SQ really excellent. Of course it all depends on the quality of parts installed. The Kaiseis were the first I tried and I think they complement the sound very well by making it sound fuller and detailed yet still warm sounding. I tried Nichicons in my other amp but not the ones you used.
Also as I had one spare Jupiter copper foil, paper and wax cap from my last mods I got another to put into the APPJ for coupling, so again top quality parts. OK so I ended up doubling the cost of the amp but at around $150 for the stock amp it is an incredible bargain IMO.
Edit: try to get a non polar cathode bypass cap if you can, better for constant power flow and hence sound.
That cathode bypass calculator I linked to is very interesting, I haven't tried it with this amp because I didn't know some of the inputs. But, for example the frequency curve for a 1000uF cap shows suspected overkill but, as there is no oscillation in the APPJ, I have stuck with it because of the excellent sound and power handling. In the cathode bypass calculator example above there was no difference in the frequency curve for a 500uF or 1000uF cap hence suspected overkill.
I tried initially with 1000uF in this amp and found as suspected that the sound was heavy, dull and the higher frequency sparkle were lost. As soon as I installed the smaller Audyn True Copper caps as bypasses the sound became very lively again. Without that I suspect that something around 500uF might suffice on its own.
I haven't found the bass overpowering or boomy at all on the contrary it is sharp, detailed and in proportion to the rest of the frequency band. The treble also is extended and I find the whole SQ really excellent. Of course it all depends on the quality of parts installed. The Kaiseis were the first I tried and I think they complement the sound very well by making it sound fuller and detailed yet still warm sounding. I tried Nichicons in my other amp but not the ones you used.
Also as I had one spare Jupiter copper foil, paper and wax cap from my last mods I got another to put into the APPJ for coupling, so again top quality parts. OK so I ended up doubling the cost of the amp but at around $150 for the stock amp it is an incredible bargain IMO.
Edit: try to get a non polar cathode bypass cap if you can, better for constant power flow and hence sound.
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shunt-pot input resistor experiments:
(bugle-boy12ax7; 6f6s; 0.44uf FT3 as interstage; 220uf Nichicon ES as cathode bypass; 32 ohm beyer driver)
--AudioNote, non-magnetic: fine, sweet and clean; has some bite; not overly "cheerful"
--Vishay VAR: warm and slurry, bass too thick
--Caddock TF020: bass bottoms out?
--shinkoh: soft and pleasant; some boominess in bass; no bite
-- riken: clear top, more bite than AN, emotion about the same as AN but not as much fine details?
Riken's detail level is about the same as shinkoh.
AN's detail level is about the same as VAR and TF020 (and one notch above Riken and Shinkoh)
==> start with AN; if want darker use Shinkoh, if want brighter use riken.
In my other stuff, especially the electrostatics, I usually prefer TF020 or VAR, the AN has always been a bit too "sweet and cheerful". But in this amp AN is the winner.
(bugle-boy12ax7; 6f6s; 0.44uf FT3 as interstage; 220uf Nichicon ES as cathode bypass; 32 ohm beyer driver)
--AudioNote, non-magnetic: fine, sweet and clean; has some bite; not overly "cheerful"
--Vishay VAR: warm and slurry, bass too thick
--Caddock TF020: bass bottoms out?
--shinkoh: soft and pleasant; some boominess in bass; no bite
-- riken: clear top, more bite than AN, emotion about the same as AN but not as much fine details?
Riken's detail level is about the same as shinkoh.
AN's detail level is about the same as VAR and TF020 (and one notch above Riken and Shinkoh)
==> start with AN; if want darker use Shinkoh, if want brighter use riken.
In my other stuff, especially the electrostatics, I usually prefer TF020 or VAR, the AN has always been a bit too "sweet and cheerful". But in this amp AN is the winner.
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baronbeehive
Headphoneus Supremus
Don't know if you are interested but someone on another thread has very recently done a test of several resistor types I will quote here just in case:
"the AN Niobium ... very good clarity and detail over the FR, a very refined, detailed and smooth high end kind of sound... Afterwards I still tried the AN tantalums, but they were clearly worse than the niobiums with soft bass and muddied low treble. So as they now worked in my system I'd rank them as follows: First, Audio Note Niobium non-magnetic - these are worth their price here and they are in a different league to the rest. Second, the Kiwames - they have good punchy and deep bass and are relatively cheap too even if they don't have the very best clarity. Third, the AN tantalums which had many things in common with the Kiwames but were worse. Fourth the TKD metal films which have a crisp character but are low on bass. Fifth and last the Ohmite Audio Golds which didn't really stand out with their characteristics."
AN Niobium, interesting ... Partsconnexion doesn't carry them. I might try to find some one of these days.
I do not agree with Kiwame being better than AN tantalum (as series resistor in shunt-pots). Not even close.
in the PA1502A, I think the shinkoh is good enough, AN is kind of an over-kill.
... Partsconnexion doesn't carry them. I might try to find some one of these days.I do not agree with Kiwame being better than AN tantalum (as series resistor in shunt-pots). Not even close.
in the PA1502A, I think the shinkoh is good enough, AN is kind of an over-kill.
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