Bellari VP129
May 26, 2008 15:32:41 GMT
Post by rickcr42 on May 26, 2008 15:32:41 GMT
Someone in the past was asking about the circuit topology of this phono stage,an answer to which I gave a "best guess" answer that went on the devices spec sheet so I made certain assumptions
1-There being a single tube present and it being a High-MU triode I pretty much knew it was not being used as a "tube buffer" for the phono circuit as some surmised but was in fact used for gain either whole or in part
2-the stated output being very low and with the output z of a 12AX7 being high even IF used as a Cathode Follower my guess was the (stated part) Op-Amp (s) were for output stage cable driving/headphone driving/stage buffering
3-I guessed the dedicated headphone output was either a special high current buffer chip,a high current output op-amp or was just a "switch position" on the output op-amp that chose either line or headphone out
Well i was close in some way off in others
Having just come across the schema by accident and looking at it right now the 'topology" of the Bellari VP129 Phono Stage goes :
1-12AX7 High-MU Dul Triode provides first stage gain for both channels (1/2 12AX7 Tube per side).This stage is a single-ended Class-A zero NFB x30 (approx.) straight gain stage (inverting)
2-The second stage is a passive RC RIAA EQ network
3-STAGE 3 is two dual Op-Amps with 1/2 used for each channel and is a straight X10 non-inverting gain stage
4-The fourth stage is a three-pole -18dB per octave @ 20Hz subsonic filter and uses the other half of the dual op-amp used in that channel.Because this filter can be switched out the true low end response of the circuit will be determined by the final output coupling capacitor value into the load of the next stage
5-finally the output section-this is a "split" section having a dedicated "line amp" and "headphone amp" even though using 1/2 of the same dual op-amp for each with the only real differences being the output coupling section :
Line Output-10uF couling cap loaded with a shunt position 47K resistor then a 1K series resistor to decouple the interconnects from the output.The line out also has a "mute" switch for when headphones are in use (smart,very smart
)
Headphone Output-because this output is meant to drive a lower impedance than the line output and in order to maintain good fidelity at the line outs without comprimising headphone response from the same outputs,the reason for TWO stages (though I'm thinking one would have done with the OUTPUT RC switchable to intended load) this output has a 10 ohm series resistor followed by a 47uF cap which is in turn direct coupled to the TRS jack.(this value BTW tells me the lower limit for headphone impedance is around 250 ohms so if you want to use lower impedance cans add an external DC coupled buffer or increase the size of the cap )
Here is the weird part.The line outs are inverted which makes the input-to-output phase NON inverted (two inverting stages) but the headphone output is NON inverted so the phase at the TRS headphone jack IS inverted from the input signal.Why i have no clue but there must be a reason or why do it ?
THE POWER SUPPLY :
The AC is full wave rectified,filtered with a 2,200uF cap then regulated to +12 VDC which is used for both V+ and Vbias (single supply to the op-amp ) AND for the 12AX7 heater supply
The high voltage section to the 12AX7 uses a diode "voltage doubler" type supply which is in turn series-pass transistor regulated to 125 VDC ("starved Plate" supply which makes for a warmer sound on balance,just the ticket for AT cartridges )
So that is the scoop and all in all,pretty damn nice for the amount of loot but as always when you try for a market "niche" you have to cut costs somewhere and in this case my guess would be in the "usual suspects" areas of the passive parts,the power supply and "features"
First the passives.At the input is a 47K cartridge load resistor.Without KNOWING what it is my guess is a generic metal film is in play and if so should be tossed and replaced with a better sounding precision part (naked vishay or such).
The Passive RIAA EQ Network i would NOT TOUCH unless you know for absolutely sure you can replace the parts with EXACT VALUE replacements or you will screw up the "tone" of the amp.It is the RIAA curve that mostly determines the SQ qualities of any phono stage and if you mess with it you may lose the parts of the sound you like so easy bubba
The subsonic filter is a no brainer.The parts being pretty much standard values and again likely generic/consumer grade parts it may be a good idea to replace the RC filter parts with left/right matched sets of the same values using film caps and precision resistors (though not generic metal films which will add a "hard" or "brittle" edge to everything !)
Output networks :
Film caps baby ! The best you can Fit/Afford this being a serious choke point as is true with any device using AC coupling.as for the resistors ? Whatever you like.This is not a "precision part" circuit position but it IS a "everything goes through it" SQ effecting position so personally i would shoot to a Riken or Allen Bradley Carbon comp (both 1 Watters ) but that is personal choice so whatever floats your boat
As for the power supply ?
Nothing unless you have issues with noise or ripple (especially at the heater position) and if so then I would increase the 2,200uF to 4,700 if it will fit (add some current limiting/surge protection resistors between IT and the rectifiers),would "snubberize" the full wave rectifier section and maybe even use better diodes,would add small bypass caps across each of the "voltage doubler" 47uF parallel caps and would add a reverse voltage protection diode across the 12 V-Reg chip.
Ideally ? Lose the entire power supply section and build a "proper" external supply with a TRUE high voltage DC supply,a dedicated heater supply and a "dual" 12 VDC Op-Amp supply thus losing the "Vbias" connections to the op-amps but that would cost as much or more than the entire product.
An improvement but less than ideal though far cheaper ?
First of,a higher current wall wart to power :
a dedicated 12 VDC heater supply tapped before the single 12V V-Reg chip and for the +12 VDC Op-Amp feed would TRY to weasle in a second 2,200uf filter cap separated from the first by a series resistor of 100 ohms creating a CRC DC Filter to stomp any possible ripple down before it can play hell with the audio but all in all,a pretty cool product that fits into it OWN "niche" having no real competition at that price point for what it does and that "what it does" probably includes about the best damn input overload/"pop and click" handling you will find at anything near the price due to the 12AX7 input being WAY better than any monolithic device could be if at the same high gain level there just not being enough headroom at "12VDC" or even 15,or 24 or...........when compared to 125 VDC.
FEED ME BAYBEEE !
rick out
1-There being a single tube present and it being a High-MU triode I pretty much knew it was not being used as a "tube buffer" for the phono circuit as some surmised but was in fact used for gain either whole or in part
2-the stated output being very low and with the output z of a 12AX7 being high even IF used as a Cathode Follower my guess was the (stated part) Op-Amp (s) were for output stage cable driving/headphone driving/stage buffering
3-I guessed the dedicated headphone output was either a special high current buffer chip,a high current output op-amp or was just a "switch position" on the output op-amp that chose either line or headphone out
Well i was close in some way off in others
Having just come across the schema by accident and looking at it right now the 'topology" of the Bellari VP129 Phono Stage goes :
1-12AX7 High-MU Dul Triode provides first stage gain for both channels (1/2 12AX7 Tube per side).This stage is a single-ended Class-A zero NFB x30 (approx.) straight gain stage (inverting)
2-The second stage is a passive RC RIAA EQ network
3-STAGE 3 is two dual Op-Amps with 1/2 used for each channel and is a straight X10 non-inverting gain stage
4-The fourth stage is a three-pole -18dB per octave @ 20Hz subsonic filter and uses the other half of the dual op-amp used in that channel.Because this filter can be switched out the true low end response of the circuit will be determined by the final output coupling capacitor value into the load of the next stage
5-finally the output section-this is a "split" section having a dedicated "line amp" and "headphone amp" even though using 1/2 of the same dual op-amp for each with the only real differences being the output coupling section :
Line Output-10uF couling cap loaded with a shunt position 47K resistor then a 1K series resistor to decouple the interconnects from the output.The line out also has a "mute" switch for when headphones are in use (smart,very smart
)Headphone Output-because this output is meant to drive a lower impedance than the line output and in order to maintain good fidelity at the line outs without comprimising headphone response from the same outputs,the reason for TWO stages (though I'm thinking one would have done with the OUTPUT RC switchable to intended load) this output has a 10 ohm series resistor followed by a 47uF cap which is in turn direct coupled to the TRS jack.(this value BTW tells me the lower limit for headphone impedance is around 250 ohms so if you want to use lower impedance cans add an external DC coupled buffer or increase the size of the cap
)Here is the weird part.The line outs are inverted which makes the input-to-output phase NON inverted (two inverting stages) but the headphone output is NON inverted so the phase at the TRS headphone jack IS inverted from the input signal.Why i have no clue but there must be a reason or why do it ?
THE POWER SUPPLY :
The AC is full wave rectified,filtered with a 2,200uF cap then regulated to +12 VDC which is used for both V+ and Vbias (single supply to the op-amp
) AND for the 12AX7 heater supplyThe high voltage section to the 12AX7 uses a diode "voltage doubler" type supply which is in turn series-pass transistor regulated to 125 VDC ("starved Plate" supply which makes for a warmer sound on balance,just the ticket for AT cartridges
)So that is the scoop and all in all,pretty damn nice for the amount of loot but as always when you try for a market "niche" you have to cut costs somewhere and in this case my guess would be in the "usual suspects" areas of the passive parts,the power supply and "features"
First the passives.At the input is a 47K cartridge load resistor.Without KNOWING what it is my guess is a generic metal film is in play and if so should be tossed and replaced with a better sounding precision part (naked vishay or such).
The Passive RIAA EQ Network i would NOT TOUCH unless you know for absolutely sure you can replace the parts with EXACT VALUE replacements or you will screw up the "tone" of the amp.It is the RIAA curve that mostly determines the SQ qualities of any phono stage and if you mess with it you may lose the parts of the sound you like so easy bubba
The subsonic filter is a no brainer.The parts being pretty much standard values and again likely generic/consumer grade parts it may be a good idea to replace the RC filter parts with left/right matched sets of the same values using film caps and precision resistors (though not generic metal films which will add a "hard" or "brittle" edge to everything !)
Output networks :
Film caps baby ! The best you can Fit/Afford this being a serious choke point as is true with any device using AC coupling.as for the resistors ? Whatever you like.This is not a "precision part" circuit position but it IS a "everything goes through it" SQ effecting position so personally i would shoot to a Riken or Allen Bradley Carbon comp (both 1 Watters
) but that is personal choice so whatever floats your boatAs for the power supply ?
Nothing unless you have issues with noise or ripple (especially at the heater position) and if so then I would increase the 2,200uF to 4,700 if it will fit (add some current limiting/surge protection resistors between IT and the rectifiers),would "snubberize" the full wave rectifier section and maybe even use better diodes,would add small bypass caps across each of the "voltage doubler" 47uF parallel caps and would add a reverse voltage protection diode across the 12 V-Reg chip.
Ideally ? Lose the entire power supply section and build a "proper" external supply with a TRUE high voltage DC supply,a dedicated heater supply and a "dual" 12 VDC Op-Amp supply thus losing the "Vbias" connections to the op-amps but that would cost as much or more than the entire product.
An improvement but less than ideal though far cheaper ?
First of,a higher current wall wart to power :
a dedicated 12 VDC heater supply tapped before the single 12V V-Reg chip and for the +12 VDC Op-Amp feed would TRY to weasle in a second 2,200uf filter cap separated from the first by a series resistor of 100 ohms creating a CRC DC Filter to stomp any possible ripple down before it can play hell with the audio but all in all,a pretty cool product that fits into it OWN "niche" having no real competition at that price point for what it does and that "what it does" probably includes about the best damn input overload/"pop and click" handling you will find at anything near the price due to the 12AX7 input being WAY better than any monolithic device could be if at the same high gain level there just not being enough headroom at "12VDC" or even 15,or 24 or...........when compared to 125 VDC.
FEED ME BAYBEEE !
rick out
)
