I'm following some written instructions as i slowly modify two Roberts/Akai mono block amps to become mic preamps. I've been told to convert the unbalanced phone jack output to a balanced XLR output by disconnecting the original phone jack wiring, installing a 500 ohm resistor on the signal output side, followed by a 1:1 ratio output transformer such as a Jensen unit. I'm told that the signal will still be too large at this point and that i will need to install an H-pad as most modern day mixers can't handle such a high voltage signal. My question is why can't i just install something like a 10K Alps Blue velvet poteniometer on the output side immediately after the transformer to control output level? I can buy them, I can't buy an H-pad.
If you are converting a single ended output to a balanced output, then you must use a balanced attenuator like an H pad (also known as a balanced T) where an L pad is a single ended device. An H pad maintains a constant impedance on both the input and output side. An L pad is constant only in one direction. (input)
When you go to professional balanced circuits you must do things that comply with the system requirements. Trying to use a dual L pad would lower the output but you could easily experience frequency aberrations from the mismatch.
One alternative would be to wire in a fixed H style attenuator using resistors. Then wire in a DPDT toggle switch to jump them if/when not needed.
A second alternative would be to somehow lower the gain before the transformer.
Victor
When you go to professional balanced circuits you must do things that comply with the system requirements. Trying to use a dual L pad would lower the output but you could easily experience frequency aberrations from the mismatch.
One alternative would be to wire in a fixed H style attenuator using resistors. Then wire in a DPDT toggle switch to jump them if/when not needed.
A second alternative would be to somehow lower the gain before the transformer.
Victor
Do you know how much you'll need to reduce the signal level?
You could use a 150 x 4 output transformer wired 2:1 which would reduce the signal level by half (-6dB).
se
You could use a 150 x 4 output transformer wired 2:1 which would reduce the signal level by half (-6dB).
se
limey222 said:
Depends.
What sort of load can the output of the amp drive? Can it handle a 600 ohm load? If so, you could use a 600 ohm T attenuator. Otherwise if it wants to see a high impedance load, then that won't work so well as the output impedance will be rather too high for the output transformer.
se
Aaaaargh!
Can I say this once and for all? 600 Ohms went out with the Ark! Professional audio expects to see low source and high destination impedances. Microphones are typically 200 Ohm source, microphone amplifiers are a maximum of 13.6k (2 x 6k8 for the phantom power) input.
Balance is crucial. If you have to use attenuators, they must be balanced T or pi.
Can I say this once and for all? 600 Ohms went out with the Ark! Professional audio expects to see low source and high destination impedances. Microphones are typically 200 Ohm source, microphone amplifiers are a maximum of 13.6k (2 x 6k8 for the phantom power) input.
Balance is crucial. If you have to use attenuators, they must be balanced T or pi.
Re: Aaaaargh!
True enough.
But the question was if he could put the attenuator ahead of the output transformer. And in that position, there are two things to consider, the load seen by the output stage, and the source impedance seen by the output transformer.
Since the output transformer doesn't want to be driven by much more than about 600 ohms, then the output would need to be able to drive about 600 ohms.
But that would only be for a T or H pad which have the same input and output impedances. I just noticed he said L pad, and in that case, he could use a shunt resistor of about 600 ohms and adjust the value of the series resistor until he achieves sufficient attenuation.
They wouldn't need to be balanced if they're going to be placed between his existing unbalanced output and the output transformer.
se
EC8010 said:
True enough.
But the question was if he could put the attenuator ahead of the output transformer. And in that position, there are two things to consider, the load seen by the output stage, and the source impedance seen by the output transformer.
Since the output transformer doesn't want to be driven by much more than about 600 ohms, then the output would need to be able to drive about 600 ohms.
But that would only be for a T or H pad which have the same input and output impedances. I just noticed he said L pad, and in that case, he could use a shunt resistor of about 600 ohms and adjust the value of the series resistor until he achieves sufficient attenuation.
They wouldn't need to be balanced if they're going to be placed between his existing unbalanced output and the output transformer.
se
Re: Re: Aaaaargh!
OK, now i might be getting somewhere, in my original post i had already stated that I had been instructed to install a 500 Ohm resistor inseries between the output stage and the output transformer. I assume that the "shunt" resistor mentioned is in addition to that? Is that installed in a different location or is it one of the same thing. BTW, I'm going to use a Jensen JT-11-DMCF which should drive 600 ohm loads to level up to +22dbu @20HZ according to the manufacturer's specs. I assume that I was told to use the 500 ohm resistor to knock-down the signal level going into the transformer since even at 1:1 the transformer output will be too high for me to effectively utilize for anything in my studio except to drive speakers, it's certainly too high to drive a mixer board input or a studio quality sound card.
What I want to do here is to be able to adjust how hard I drive to amplification stage of the mono blocks to give me varying amounts of tonal control over the signal, (read small amounts of pleasant tube distortion) but then also control in a variable manner, the output being presented to the mixer so that I don't overload the mixer input. It seems to me that using a fixed manner to know down the signal level is a bit bit hit and miss. Is there no variable method to control output available?
Please remember that I am new to all this so go slowly and be as specific as you can...thanks
Steve Eddy said:
OK, now i might be getting somewhere, in my original post i had already stated that I had been instructed to install a 500 Ohm resistor inseries between the output stage and the output transformer. I assume that the "shunt" resistor mentioned is in addition to that? Is that installed in a different location or is it one of the same thing. BTW, I'm going to use a Jensen JT-11-DMCF which should drive 600 ohm loads to level up to +22dbu @20HZ according to the manufacturer's specs. I assume that I was told to use the 500 ohm resistor to knock-down the signal level going into the transformer since even at 1:1 the transformer output will be too high for me to effectively utilize for anything in my studio except to drive speakers, it's certainly too high to drive a mixer board input or a studio quality sound card.
What I want to do here is to be able to adjust how hard I drive to amplification stage of the mono blocks to give me varying amounts of tonal control over the signal, (read small amounts of pleasant tube distortion) but then also control in a variable manner, the output being presented to the mixer so that I don't overload the mixer input. It seems to me that using a fixed manner to know down the signal level is a bit bit hit and miss. Is there no variable method to control output available?
Please remember that I am new to all this so go slowly and be as specific as you can...thanks
Re: Re: Re: Aaaaargh!
Essentially, yes.
Basically it would be two resistors tied together, with the 600 ohm across the transformer primary and the series resistor tied to the top of the primary and the other end fed from the output of the amplifier.
Have you purchased the Jensen yet? If not, as much as I love the folks at Jensen, I'd recommend going with CineMag instead. Their transformers are every bit as good as the Jensens but cost significantly less.
Not sure why you were told to use the 500 ohm resistor. It may have been for the reason you state, or the person may have been under the mistaken impression that the transformer needs to be driven with a 600 ohm source impedance.
By the way, you say these are "mono block amps." When I hear "mono block amps" I think power amplifiers. Are these in fact power amplifiers?
What would be most helpful here would be to know exactly what you have, what sort of voltage gain they have, and ideally the schematics.
There may be other, better solutions to what you want to achieve here.
Well, the series resistor could be a potentiometer which would allow the attenuation to be variable. But before going too far down that road, let's find out exactly what you've got.
I said
The...
Series...
Resistor...
Could...
Be...
😀
se
limey222 said:
Essentially, yes.
Basically it would be two resistors tied together, with the 600 ohm across the transformer primary and the series resistor tied to the top of the primary and the other end fed from the output of the amplifier.
Have you purchased the Jensen yet? If not, as much as I love the folks at Jensen, I'd recommend going with CineMag instead. Their transformers are every bit as good as the Jensens but cost significantly less.
Not sure why you were told to use the 500 ohm resistor. It may have been for the reason you state, or the person may have been under the mistaken impression that the transformer needs to be driven with a 600 ohm source impedance.
By the way, you say these are "mono block amps." When I hear "mono block amps" I think power amplifiers. Are these in fact power amplifiers?
What would be most helpful here would be to know exactly what you have, what sort of voltage gain they have, and ideally the schematics.
There may be other, better solutions to what you want to achieve here.
Well, the series resistor could be a potentiometer which would allow the attenuation to be variable. But before going too far down that road, let's find out exactly what you've got.
I said
The...
Series...
Resistor...
Could...
Be...
😀
se
Re: Re: Re: Re: Aaaaargh!
I have no circuit diagrams just the text of the original ebay listing copied below:
"Offered at auction are a matched pair of classic Roberts model 770X monoblock all tube combination microphone preamps / power amplifiers in superb condition. NOTE: These are not the usual pair of stock units requiring extensive service and modification, but instead are fully serviced, professionally customized and blueprinted examples of these popular amps. As a retired old school recording studio service/calibration tech, with decades of experience in designing custom tube preamplifiers for demanding studio applications, I fortunately still have provision for the (fairly extensive) setup and calibration procedures required for proper reconfiguration and bench testing of these models. For those not familiar with these early mini rack units, first used in the excellent sounding reel to reel recorders marketed both as Akai and Roberts, and using the same basic circuitry in Models such as the 990, 770 X, M-8, etc., they have long been very popular for use as stand alone Mic preamps in both live and recording venues. The original circuit topography, and superior build quality, allowing both microphone, as well as normal line inputs, making them ideal for tweaking and modification.
First the original 6267 triode/pentode tube and related circuitry (used for the massive equalization and current requirements in the NAB tape recording and playback head circuits of the recorders) has been removed. This section, as well as the related 6AL5 tube used in the erase and bias section (also removed) effectively have nothing to do with the use of these units as designated, only adding noise and sonic degradation if left installed. What remains is the essence of classic Single Ended tube elegance, a huge power supply transformer, supplying well filtered B+ voltage and current reserves, tube rectified (using the proven 6X4 rectifier) to the remaining SE operated dual triode high gain and driver stages for the single 6BQ5/ EL-84 SE output pentodes, operated in pure class A tetrode mode. All superfluous wiring, cabling, etc originally installed for dedicated open reel interconnection, again has been removed, with only the front panel input and output jacks (standard 1/4 inch phone type) and separate, polarity reversable standard line cords (allowing true monoblock configuration) remaining, for easy, user friendly operation. All original power supply and active circuit components have been separately tested for value and tolerance with any required replacements done using original Akai Japanese exact equivalents. All critical coupling and bypass capacitors have been replaced using bench tested and VERY closely matched NOS high voltage American Sprague Black Beauty types, proven in this circuitry for their low noise, and low DC leakage capability. The single remaining variable potentiometer, (the critical volume/gain pot for both units) has been replaced with quality early generation Alps, again, hand measured for value, low noise, and tracking linearity. These are exceptionally quiet circuits in operation, with no prevalent PSU 60HZ, 120Hz, or higher order components, and only extremely low level, wide band (pink noise) measurable, but totally non intrusive in normal operation. This residual noise floor (common resistor and tube micro hiss) is much lower than is normally found in even many high end studio and home pre amps. The large blackface upgraded lighted analog metering, has been calibrated, with superb accuracy, channel tracking (better than 1/4 Db full scale) and currently showing 0 Db at the very onset of soft natural tube compression, and +2 Db at threshold of soft (and uniform) clipping. No Tube amplifier is any better than the components used in it's design. The phenomenal output transformers supplied in these early models is superior in performance in every measurable and audible parameter. Power output waveforms including demanding square wave analysis, reveal oscilloscope traces virtually unseen in lesser SE designs, with superb full bandwidth linearity, no ringing or overshoot even when driven into soft clipping, and transient response rivaling the best of P/P circuits. These are very good sounding amps, capable of all of the sonic virtues associated with Single Ended mythos, and seemingly none of the (often not spoken of) shortcomings. The properly biased Class A outputs, (Unbypassed Cathode bias) allow the supplied (low hour, matched used USA GE 6BQ5/ EL- 84s to couple through the 8 ohm rated transformers in such a way that appears uncanny in my experience. Square wave output wave forms are virtually uniform in linearity, bandwidth and harmonic detail from below 4 ohm dummy loads to open circuit conditions, with electronic and thermal stability unchanged regardless of load impedance or reactance. The 12AD7/12AX7 class of dual triodes specified for use in these circuits allows for a vast range of tube substitution tweaking. In my experience the finest design for use as HiFi amps, (with equally low noise, and bandwidth capability in high gain mic use, is the venerable British Mullard ECC83/ 12AX7, and as such these are supplied with low hour used and perfectly matched examples with intact silkscreening, ready to use. The supplied rectifiers are a matched pair of the proven Sylvania USA black plate 6X4. I will try to sum up this admittedly over wrought description: The multi position slide switch used originally to switch between record and play modes for the recorder is securely bolted into the correct fixed position. The cut type tone control is of course, no longer active and the original knobs are configured in this way. The first (left knob) controls volume/gain and the right set up operate the two position control, orig. used for speed equalization switching. As currently modified, this switch now allows use in one position for absolute flat response, with a second option giving a low order rise in high frequency response (begining at just below 11 Khz to a maximum peak of +4 Db at 17 Khz). Experience has shown over the years, that this relatively subtle curve is ideal for touch up of mic response from a large number of vintage designs, allowing just the right increase for miking higher detail for Piano, cymbal, etc. harmonics, from less than ideal microphone sources. This slight (but distinctly) noticable response change, also serves very well to compensate for the often time, too soft high end response of vintage full range and tweeter loudspeaker designs, the use of which is mandated by the efficiency requirements of the 4 watt RMS + output of these amps.
I have purposely left the input and output circuitry (and operating level) options for mic pre use, up to the potential high bidder. The Mic inputs are currently set up for use only with high Z unbalanced mics (with standard 1/4" jacks). The conversion to balanced input is of course, easily facilitated by use of any common Low Z (600 ohm) balanced to grid input transformer (either externally or internally mounted) and the required XLR or TRS jack for connection. Traditionally the line outputs for these are optimized by use of the power output section (again 1/4" phone jack) for interface. The superb wide bandwidth linearity and stability into practically any load impedance, makes this configuration possible.This allows for the ideal current source, low impedance output capability to be used as is (for unbalanced input to vintage line level boards and mixers, etc.) and attenuated using the receiving units stepped or variable pad or input operating level attenuator. If lower source voltage is required only the addition of a simple fixed or variable voltage divider network is required to preset output and (purely resistive source impedance) to any standard operating level from extremes such as -20 db to classic +10 db levels. For balanced output use, again, a matching transformer and jacks are required, but the low output impedance provided, allows for use of the much less costly (compared to quality plate to balanced line) low Z to balanced low Z designs (600 to bal. 600 works nicely). These narrow faceplates allow for use of the remaining EIA rack width (when used with rack extensions) as excellent patch bay panels for installation of the desired components and jacks for custom setups.
For the recording engineer, home HiFi purest, or anyone desiring the versatility, and audio capability of these lovely little monoblocks, without the extravagant price demanded for the modified Designer capped Beautique versions (which are probably quite nice), but perhaps not worth the many hundreds each, often charged, here may well be an excellent opportunity. I am selling these Roberts units as a pair with NO RESERVE. "
So yes, they are mono block amps and yes, i do want to finish off the conversion and make the best of it since I have all the necessay components.
So should I use a 600 ohm shunt resistor across the primary of the output transformer and then use the Alps Blue velvet 100K pot as my series resistor between the amp output and the top of the primary? If not what fixed value should I make the series resistor ?
If I decided not to make the output balanced, and retained the present unbalanced setup and just used a series resistor and the Alps pot (L-pad) to knock down the signal level, would I likely still have a viable mic pre as long as i kept the output line short to the mixer?
I have no circuit diagrams just the text of the original ebay listing copied below:
"Offered at auction are a matched pair of classic Roberts model 770X monoblock all tube combination microphone preamps / power amplifiers in superb condition. NOTE: These are not the usual pair of stock units requiring extensive service and modification, but instead are fully serviced, professionally customized and blueprinted examples of these popular amps. As a retired old school recording studio service/calibration tech, with decades of experience in designing custom tube preamplifiers for demanding studio applications, I fortunately still have provision for the (fairly extensive) setup and calibration procedures required for proper reconfiguration and bench testing of these models. For those not familiar with these early mini rack units, first used in the excellent sounding reel to reel recorders marketed both as Akai and Roberts, and using the same basic circuitry in Models such as the 990, 770 X, M-8, etc., they have long been very popular for use as stand alone Mic preamps in both live and recording venues. The original circuit topography, and superior build quality, allowing both microphone, as well as normal line inputs, making them ideal for tweaking and modification.
First the original 6267 triode/pentode tube and related circuitry (used for the massive equalization and current requirements in the NAB tape recording and playback head circuits of the recorders) has been removed. This section, as well as the related 6AL5 tube used in the erase and bias section (also removed) effectively have nothing to do with the use of these units as designated, only adding noise and sonic degradation if left installed. What remains is the essence of classic Single Ended tube elegance, a huge power supply transformer, supplying well filtered B+ voltage and current reserves, tube rectified (using the proven 6X4 rectifier) to the remaining SE operated dual triode high gain and driver stages for the single 6BQ5/ EL-84 SE output pentodes, operated in pure class A tetrode mode. All superfluous wiring, cabling, etc originally installed for dedicated open reel interconnection, again has been removed, with only the front panel input and output jacks (standard 1/4 inch phone type) and separate, polarity reversable standard line cords (allowing true monoblock configuration) remaining, for easy, user friendly operation. All original power supply and active circuit components have been separately tested for value and tolerance with any required replacements done using original Akai Japanese exact equivalents. All critical coupling and bypass capacitors have been replaced using bench tested and VERY closely matched NOS high voltage American Sprague Black Beauty types, proven in this circuitry for their low noise, and low DC leakage capability. The single remaining variable potentiometer, (the critical volume/gain pot for both units) has been replaced with quality early generation Alps, again, hand measured for value, low noise, and tracking linearity. These are exceptionally quiet circuits in operation, with no prevalent PSU 60HZ, 120Hz, or higher order components, and only extremely low level, wide band (pink noise) measurable, but totally non intrusive in normal operation. This residual noise floor (common resistor and tube micro hiss) is much lower than is normally found in even many high end studio and home pre amps. The large blackface upgraded lighted analog metering, has been calibrated, with superb accuracy, channel tracking (better than 1/4 Db full scale) and currently showing 0 Db at the very onset of soft natural tube compression, and +2 Db at threshold of soft (and uniform) clipping. No Tube amplifier is any better than the components used in it's design. The phenomenal output transformers supplied in these early models is superior in performance in every measurable and audible parameter. Power output waveforms including demanding square wave analysis, reveal oscilloscope traces virtually unseen in lesser SE designs, with superb full bandwidth linearity, no ringing or overshoot even when driven into soft clipping, and transient response rivaling the best of P/P circuits. These are very good sounding amps, capable of all of the sonic virtues associated with Single Ended mythos, and seemingly none of the (often not spoken of) shortcomings. The properly biased Class A outputs, (Unbypassed Cathode bias) allow the supplied (low hour, matched used USA GE 6BQ5/ EL- 84s to couple through the 8 ohm rated transformers in such a way that appears uncanny in my experience. Square wave output wave forms are virtually uniform in linearity, bandwidth and harmonic detail from below 4 ohm dummy loads to open circuit conditions, with electronic and thermal stability unchanged regardless of load impedance or reactance. The 12AD7/12AX7 class of dual triodes specified for use in these circuits allows for a vast range of tube substitution tweaking. In my experience the finest design for use as HiFi amps, (with equally low noise, and bandwidth capability in high gain mic use, is the venerable British Mullard ECC83/ 12AX7, and as such these are supplied with low hour used and perfectly matched examples with intact silkscreening, ready to use. The supplied rectifiers are a matched pair of the proven Sylvania USA black plate 6X4. I will try to sum up this admittedly over wrought description: The multi position slide switch used originally to switch between record and play modes for the recorder is securely bolted into the correct fixed position. The cut type tone control is of course, no longer active and the original knobs are configured in this way. The first (left knob) controls volume/gain and the right set up operate the two position control, orig. used for speed equalization switching. As currently modified, this switch now allows use in one position for absolute flat response, with a second option giving a low order rise in high frequency response (begining at just below 11 Khz to a maximum peak of +4 Db at 17 Khz). Experience has shown over the years, that this relatively subtle curve is ideal for touch up of mic response from a large number of vintage designs, allowing just the right increase for miking higher detail for Piano, cymbal, etc. harmonics, from less than ideal microphone sources. This slight (but distinctly) noticable response change, also serves very well to compensate for the often time, too soft high end response of vintage full range and tweeter loudspeaker designs, the use of which is mandated by the efficiency requirements of the 4 watt RMS + output of these amps.
I have purposely left the input and output circuitry (and operating level) options for mic pre use, up to the potential high bidder. The Mic inputs are currently set up for use only with high Z unbalanced mics (with standard 1/4" jacks). The conversion to balanced input is of course, easily facilitated by use of any common Low Z (600 ohm) balanced to grid input transformer (either externally or internally mounted) and the required XLR or TRS jack for connection. Traditionally the line outputs for these are optimized by use of the power output section (again 1/4" phone jack) for interface. The superb wide bandwidth linearity and stability into practically any load impedance, makes this configuration possible.This allows for the ideal current source, low impedance output capability to be used as is (for unbalanced input to vintage line level boards and mixers, etc.) and attenuated using the receiving units stepped or variable pad or input operating level attenuator. If lower source voltage is required only the addition of a simple fixed or variable voltage divider network is required to preset output and (purely resistive source impedance) to any standard operating level from extremes such as -20 db to classic +10 db levels. For balanced output use, again, a matching transformer and jacks are required, but the low output impedance provided, allows for use of the much less costly (compared to quality plate to balanced line) low Z to balanced low Z designs (600 to bal. 600 works nicely). These narrow faceplates allow for use of the remaining EIA rack width (when used with rack extensions) as excellent patch bay panels for installation of the desired components and jacks for custom setups.
For the recording engineer, home HiFi purest, or anyone desiring the versatility, and audio capability of these lovely little monoblocks, without the extravagant price demanded for the modified Designer capped Beautique versions (which are probably quite nice), but perhaps not worth the many hundreds each, often charged, here may well be an excellent opportunity. I am selling these Roberts units as a pair with NO RESERVE. "
So yes, they are mono block amps and yes, i do want to finish off the conversion and make the best of it since I have all the necessay components.
So should I use a 600 ohm shunt resistor across the primary of the output transformer and then use the Alps Blue velvet 100K pot as my series resistor between the amp output and the top of the primary? If not what fixed value should I make the series resistor ?
If I decided not to make the output balanced, and retained the present unbalanced setup and just used a series resistor and the Alps pot (L-pad) to knock down the signal level, would I likely still have a viable mic pre as long as i kept the output line short to the mixer?
First let me say that after reading that auction description I am half crazy. I've never before read such a bunch of disgusting hyped-up verbiage as that. Not even in the high end audio magazine adds. That seller is a piece of work.
Secondly, many years ago I owned an Akai recorder just like that. The electronics may be decent, but the transport was a piece of garbage from a flutter and speed standpoint. ( I could explain why but that's not important here.)
If you mean the original speaker output transformer, no. A 6BQ5 cannot work into a 600 ohm plate load which is what will happen if you parallel the primary with that resistor. And the secondary must be terminated with a dummy load resistor (probably 4 ohms) if the speaker is not used in order to keep the tube properly loaded.
To be succinct, yes. But why don't you just use the line output? You may discover that it's output level is fine for driving external componets. After all, that's what it's there for.
Btw, for your edification there is a service manual available at www.hifimanuals.com
And also on eBay selling under the same name.
Victor
Secondly, many years ago I owned an Akai recorder just like that. The electronics may be decent, but the transport was a piece of garbage from a flutter and speed standpoint. ( I could explain why but that's not important here.)
If you mean the original speaker output transformer, no. A 6BQ5 cannot work into a 600 ohm plate load which is what will happen if you parallel the primary with that resistor. And the secondary must be terminated with a dummy load resistor (probably 4 ohms) if the speaker is not used in order to keep the tube properly loaded.
To be succinct, yes. But why don't you just use the line output? You may discover that it's output level is fine for driving external componets. After all, that's what it's there for.
Btw, for your edification there is a service manual available at www.hifimanuals.com
And also on eBay selling under the same name.
Victor
HollowState said:
No, I meant the new jensen output transformer that i was advised to buy for balancing the output, see previous posts. Ok , now i'm totally confused again, this is all turning into a bit of a nightmare senario with so much conflicting advice. If i keep the prsent unbalanced output, do not use the Jensen trannie, add a 500 ohm series resistor just prior to adding the Alps Blue Velvet 100K pot (that I was previously advised to use) will it present a reasonable level of signal to my external mixer that won't overload things?
Here are some final instructions that i got from the seller:
Tube preamp/Amplifier application notes:
The input and out jacks used in these modified Roberts/Akai tube
preamplifiers are as shown in the accompanying diagram. The top left two 1/4" phone jack are no longer used (still linked to the circuitry, but no longer functional). The line level input for use as stand alone power amplifiers is the top of the two remaining jacks on the left side of the lower section. This input accept any normal line level, high impedance source (outboard solid state or tube mono or stereo preamplifier, designed for home or studio use, CD player outputs, Tape outputs etc. The volume (gain) of this input (as well as the mic input) is controlled by the master
volume control as shown.
The high impedance microphone input is the 1/4" phone jack directly below this. This input accepts any unbalanced mic in the normal high impedance output range (10 Kohm to 100 Kohm) directly for use, both as stand alone mic preamp units, as well as direct pre/power monitoring (using the power amp section to drive loudspeakers). When used as either Mic pre/power, or line
level pre/power amplifiers, the meters read basically as power meters with 0 Db accurately indicating the onset of soft pre-clipping compression into any loudspeaker load.
Note: The same 1/4 " phone jack used for direct connection of the
loudspeaker (Bottom right jack below the power switch) is also now used for the unbalanced line output to ancillary equipment such as mixer boards, other effect units, or higher power amplifiers. This line output, because it functions as a constant current/low impedance source, is ideal as a low noise output to drive even extended length unbalanced lines, with little
chance of noise component pickup, even in elaborate studio
applications.
This output supplies very linear, full bandwidth (> 20 hz-20khz)
response, of any line and especially, miked source material, with almost unlimited, unclippable signal level gain. Because of the dynamic capability and massive
output voltage capability, care MUST be exercised when connecting to normal studio equipment, such as mixer consoles.
When the output of these units is fed to newer generation consoles or mixers, (Except for the best of European or British designs) it is highly likely than some sort of attenuation of the unit's output level is required.
Classic vintage studio mixers (both tube, and solid state) normally had much more elaborate and accurate onboard input selection and attenuation provision for tube levels and dynamics. Many had high quality calibrated H network, ladder type, step-attenuators for balanced inputs. For use in unbalanced or bridged input applications, the output signal from these units can easily be attenuated in the following manner:
As shown in the poor quality hand drawn diagram, the unbalanced line from the outputs (either simply plugged into the jacks, or hard wired at the same point in the circuit) can be patched to any normal L-pad attenuator, such as is commonly used in speaker load attenuation. The choice of L-pad type
and value is not critical (Any quality 8 ohm, to 32 ohm, or greater L-pad will do) and since there is no real power transfer to consider, even lower power rated types work nicely. A good quality choice is NOS earlier generation Velvet Brand (Japan) which are usually accurate and low noise in operation.
By simply wiring these L-pads into the output circuit the output signal voltage level (with no measurable sonic or bandwidth degeneration) can be preset attenuated to any desired input level required for the normal line inputs to the board,
with the metering of the pres set to show 0 VU at any vintage or modern standard operating level (0 VU = -20 to +10db ). Again Note: The Positive + lead from the L-pad to the line should have a 500 ohm resistor (preferably a low noise non carbon type) connected in series to the output. This works as a smoothing resistor for a low reactance/ purely resistive line load and
assures correct current limiting to the mixer input, maintaining a more accurate line impedance.
For balanced out configuration, this line output (either pre or post
attenuated, should be fed to the input windings of a quality matching unbalanced line to balanced transformer. Again the impedance is not terribly important, with any unbalanced 500/600 ohm primaries, and 500/600 ohm rated balanced (center tap) secondaries acceptable. With the 500 ohm
resister, again used in series with the positive input to the
transformer. If post transformer (balanced attenuation) is desired, the use of normal 200/600 ohm balanced input to 200/600 balanced out H type ladder network, step or variable attenuators work perfectly.
Microphone input impedance and level matching for use with low Z balanced mics, can be easily accommodated by any of the traditional methods. For design of permanent microphone
balanced low Z inputs, there is possibly room in the existing Roberts chassis for use of smaller sized input transformers such as the classic American UTC ouncer series, and smaller US Micro Tran and Triad models. Larger transformers and connecting jacks can be fitted to a standard outboard rack panel patch bay,
or stand alone project/patch box (both of which can also of course, contain the output attenuation/ matching networks as well). All I can add is, to look for either square or round potted, or
high grade exposed design, of proven manufacturing quality, making sure the chosen transformer is studio grade in response linearity, bandwidth, and overall sonic capability, and is the correct impedance match for the application. Unbalanced high impedance secondary windings in the range of 50 kohm to 100 kohm is ideal, with multi-tap output impedance capability, a
plus for optimum level matching. Input balanced impedance should be the standard 200 to 600 ohm (balance impedance) rating for matching to most common XLR male or TRS (tip/ring/sleeve) plug terminated microphone cables.
Your comments are welcome!
Tube preamp/Amplifier application notes:
The input and out jacks used in these modified Roberts/Akai tube
preamplifiers are as shown in the accompanying diagram. The top left two 1/4" phone jack are no longer used (still linked to the circuitry, but no longer functional). The line level input for use as stand alone power amplifiers is the top of the two remaining jacks on the left side of the lower section. This input accept any normal line level, high impedance source (outboard solid state or tube mono or stereo preamplifier, designed for home or studio use, CD player outputs, Tape outputs etc. The volume (gain) of this input (as well as the mic input) is controlled by the master
volume control as shown.
The high impedance microphone input is the 1/4" phone jack directly below this. This input accepts any unbalanced mic in the normal high impedance output range (10 Kohm to 100 Kohm) directly for use, both as stand alone mic preamp units, as well as direct pre/power monitoring (using the power amp section to drive loudspeakers). When used as either Mic pre/power, or line
level pre/power amplifiers, the meters read basically as power meters with 0 Db accurately indicating the onset of soft pre-clipping compression into any loudspeaker load.
Note: The same 1/4 " phone jack used for direct connection of the
loudspeaker (Bottom right jack below the power switch) is also now used for the unbalanced line output to ancillary equipment such as mixer boards, other effect units, or higher power amplifiers. This line output, because it functions as a constant current/low impedance source, is ideal as a low noise output to drive even extended length unbalanced lines, with little
chance of noise component pickup, even in elaborate studio
applications.
This output supplies very linear, full bandwidth (> 20 hz-20khz)
response, of any line and especially, miked source material, with almost unlimited, unclippable signal level gain. Because of the dynamic capability and massive
output voltage capability, care MUST be exercised when connecting to normal studio equipment, such as mixer consoles.
When the output of these units is fed to newer generation consoles or mixers, (Except for the best of European or British designs) it is highly likely than some sort of attenuation of the unit's output level is required.
Classic vintage studio mixers (both tube, and solid state) normally had much more elaborate and accurate onboard input selection and attenuation provision for tube levels and dynamics. Many had high quality calibrated H network, ladder type, step-attenuators for balanced inputs. For use in unbalanced or bridged input applications, the output signal from these units can easily be attenuated in the following manner:
As shown in the poor quality hand drawn diagram, the unbalanced line from the outputs (either simply plugged into the jacks, or hard wired at the same point in the circuit) can be patched to any normal L-pad attenuator, such as is commonly used in speaker load attenuation. The choice of L-pad type
and value is not critical (Any quality 8 ohm, to 32 ohm, or greater L-pad will do) and since there is no real power transfer to consider, even lower power rated types work nicely. A good quality choice is NOS earlier generation Velvet Brand (Japan) which are usually accurate and low noise in operation.
By simply wiring these L-pads into the output circuit the output signal voltage level (with no measurable sonic or bandwidth degeneration) can be preset attenuated to any desired input level required for the normal line inputs to the board,
with the metering of the pres set to show 0 VU at any vintage or modern standard operating level (0 VU = -20 to +10db ). Again Note: The Positive + lead from the L-pad to the line should have a 500 ohm resistor (preferably a low noise non carbon type) connected in series to the output. This works as a smoothing resistor for a low reactance/ purely resistive line load and
assures correct current limiting to the mixer input, maintaining a more accurate line impedance.
For balanced out configuration, this line output (either pre or post
attenuated, should be fed to the input windings of a quality matching unbalanced line to balanced transformer. Again the impedance is not terribly important, with any unbalanced 500/600 ohm primaries, and 500/600 ohm rated balanced (center tap) secondaries acceptable. With the 500 ohm
resister, again used in series with the positive input to the
transformer. If post transformer (balanced attenuation) is desired, the use of normal 200/600 ohm balanced input to 200/600 balanced out H type ladder network, step or variable attenuators work perfectly.
Microphone input impedance and level matching for use with low Z balanced mics, can be easily accommodated by any of the traditional methods. For design of permanent microphone
balanced low Z inputs, there is possibly room in the existing Roberts chassis for use of smaller sized input transformers such as the classic American UTC ouncer series, and smaller US Micro Tran and Triad models. Larger transformers and connecting jacks can be fitted to a standard outboard rack panel patch bay,
or stand alone project/patch box (both of which can also of course, contain the output attenuation/ matching networks as well). All I can add is, to look for either square or round potted, or
high grade exposed design, of proven manufacturing quality, making sure the chosen transformer is studio grade in response linearity, bandwidth, and overall sonic capability, and is the correct impedance match for the application. Unbalanced high impedance secondary windings in the range of 50 kohm to 100 kohm is ideal, with multi-tap output impedance capability, a
plus for optimum level matching. Input balanced impedance should be the standard 200 to 600 ohm (balance impedance) rating for matching to most common XLR male or TRS (tip/ring/sleeve) plug terminated microphone cables.
Your comments are welcome!
Hokay, I just got home from a big Christmas dinner and feel pretty good.
So sorry that I confused you as that was not my intent. It's just that there are two transformers involved here and I perhaps got confused myself.
Because these units have been modified I must make certain assumptions without seeing the schematic after the changes. These amplifiers had a single ended power amplifier stage that was used to drive speakers which were mounted inside the cabinet. I'm assuming that the speaker transformer output is what is being used as the low impedance output source. This point (transformer secondary) must be terminated with the characteristic resistance of the original speaker, be it four or eight ohms. And this may already be done internally. Look for that resistor somewhere inside, perhaps at the jack.
If you wish to connect your Jensen transformer to this output, then put the 500 ohm resistor in series with the Jensen transformer primary. Not in parallel. You do this to help maintain the desired load on the 6BQ5 (remember the terminating resistor) and also because the transformer may function better with a higher source then 4 to 8 ohms. 500+ ohms is still considered a low impedance source.
Btw, I looked up the specifications on your Jensen transformer and it does not provide a "true" balanced output because the secondary is not center tapped. I'd call it a quasi balanced output by referencing to chassis ground. Also, the specs do not refer to it as a balanced transformer, only a one-to-one matching transformer. But this may not be important to your purpose or use.
If you chose not to use the Jensen, then simply connecting the control across the terminated output would probably work. I don't think the 500 ohm resistor would make much difference whether it was there in series with the control or not. It would, however, serve to maintain a minimim impedance to the outside world. But I would use a lower value control, perhaps 10K ohm. Using a 100K ohm control to bridge 4 ohms won't damage anything and will function, but it's a waste of a fancy potentiometer IMHO where a regular Allen Bradley control would serve as well.
If you want to use the Jensen transformer and also be able to adjust the output level, then I would use a speaker type L or T pad, of the proper value, between the output and the Jensen transformer primary. I would also use the 500 ohm resistor between the pad and the Jensen just to keep from loading things down too much. Using your Alps pot with the Jensen is perhaps questionable as the resistance seems quite high. But you might as well try it a see how it functions.
I'm not sure what value speakers were originally used. Whether they were 4 or 8 ohms. You'll want to determin this in order to use the proper value L or T pad. And if you use the "pad" you may not need the terminating resistor (inside) because the pad will serve that purpose. The seller says that the value of a pad isn't critical. That's not really true if you want to maintain proper loading of the output stage, and/or if you keep a terminating resistor in place. By using a 500 ohm resistor before the pad, it becomes less critical. Some experimentation may be necessary.
The main thing to remember is that the internal output transformer must be terminated with it's characteristic (speaker) impedance, whether that is by a fixed resistor or a variable L or T pad. (speaker type) Keep the terminating resistor in place if you use a high value control or any series resistor.
Finally, these amps originally had a line out jack. It's unclear to me if the seller left these alone or disconnected them. Normally the signal was picked off somewhere before the power output stage and volume control and feed out to be used for monitoring through external equipment. These could still serve that purpose if they were left intact.
I hope this makes a little more sense.
Victor
So sorry that I confused you as that was not my intent. It's just that there are two transformers involved here and I perhaps got confused myself.
Because these units have been modified I must make certain assumptions without seeing the schematic after the changes. These amplifiers had a single ended power amplifier stage that was used to drive speakers which were mounted inside the cabinet. I'm assuming that the speaker transformer output is what is being used as the low impedance output source. This point (transformer secondary) must be terminated with the characteristic resistance of the original speaker, be it four or eight ohms. And this may already be done internally. Look for that resistor somewhere inside, perhaps at the jack.
If you wish to connect your Jensen transformer to this output, then put the 500 ohm resistor in series with the Jensen transformer primary. Not in parallel. You do this to help maintain the desired load on the 6BQ5 (remember the terminating resistor) and also because the transformer may function better with a higher source then 4 to 8 ohms. 500+ ohms is still considered a low impedance source.
Btw, I looked up the specifications on your Jensen transformer and it does not provide a "true" balanced output because the secondary is not center tapped. I'd call it a quasi balanced output by referencing to chassis ground. Also, the specs do not refer to it as a balanced transformer, only a one-to-one matching transformer. But this may not be important to your purpose or use.
If you chose not to use the Jensen, then simply connecting the control across the terminated output would probably work. I don't think the 500 ohm resistor would make much difference whether it was there in series with the control or not. It would, however, serve to maintain a minimim impedance to the outside world. But I would use a lower value control, perhaps 10K ohm. Using a 100K ohm control to bridge 4 ohms won't damage anything and will function, but it's a waste of a fancy potentiometer IMHO where a regular Allen Bradley control would serve as well.
If you want to use the Jensen transformer and also be able to adjust the output level, then I would use a speaker type L or T pad, of the proper value, between the output and the Jensen transformer primary. I would also use the 500 ohm resistor between the pad and the Jensen just to keep from loading things down too much. Using your Alps pot with the Jensen is perhaps questionable as the resistance seems quite high. But you might as well try it a see how it functions.
I'm not sure what value speakers were originally used. Whether they were 4 or 8 ohms. You'll want to determin this in order to use the proper value L or T pad. And if you use the "pad" you may not need the terminating resistor (inside) because the pad will serve that purpose. The seller says that the value of a pad isn't critical. That's not really true if you want to maintain proper loading of the output stage, and/or if you keep a terminating resistor in place. By using a 500 ohm resistor before the pad, it becomes less critical. Some experimentation may be necessary.
The main thing to remember is that the internal output transformer must be terminated with it's characteristic (speaker) impedance, whether that is by a fixed resistor or a variable L or T pad. (speaker type) Keep the terminating resistor in place if you use a high value control or any series resistor.
Finally, these amps originally had a line out jack. It's unclear to me if the seller left these alone or disconnected them. Normally the signal was picked off somewhere before the power output stage and volume control and feed out to be used for monitoring through external equipment. These could still serve that purpose if they were left intact.
I hope this makes a little more sense.
Victor
HollowState said:
Thanks Victor, I've been trying to attach a jpeg of the orginal hand sketch that the seller sent me but it doesn't seem to want to attach to my DIYAudio message even though the file size is small enough. If you want to see a copy please email me at lmey222@yahoo.com.
I'll look to see if the seller modded the output circuit in the manner you described. At this point if I understand you correctly I'm going to install the 500 ohm resistor in series just prior to the Alps pot. Even though the pots value may be too high, since I've already purchased it i might as well use and see if it works. The planned mods up to this point would comply with the seller's instructions on how to use the unbalnced output and achieve some form of variable attentuation (L-pad) of the output signal. If I read you right, I would then have the choice of installing the Jensen transformer after the Alps pot to give me a "quasi" balanced output while maintaining a variable attentuation of the output...is that a correct assumption based on your instructions. BTW, the sketch will probably make things clearer if I can get it to you.
OK, I traced the wiring from the unbalanced output jack (no resistor there) back to the output of the transformer. There appears to be an Akai 100 ohm resistor strapped across the out put terminals ( I suspect it may be an original component, not added later since it is an Akai) . Does that help in any way?
Yup, that's correct. I've set my personal info page to accept emails from registered users. But you may not be able to attach anything or insert pictures, so I'll send you a note and you can respond directly with the diagram.
Victor
HollowState said:
No, no. Balanced means that impedances from each leg to ground are balanced/matched. It most emphatically does not mean that the signals have a centre tap to ground. In fact, connecting the centre tap of a signal transformer to ground often increases noise pick-up. So why is the centre-tap there? It's to allow phantom circuits or DC signalling to be used on telecommunications circuits. A transformer that is wound for accurate balance (to maximise CMRR) will generally say so, and in the UK used to be known as a repeating coil (rep coil for short) from old telecommunications parlance. Typically, split bobbin coil construction is needed to achieve the balance necessary to send broadcast quality audio down hundreds of miles of multi-pair cable without picking up unacceptable levels of telegraph noise etc.
Hmmm....lemme see here. (scratches head)
Yes true, but then where is the electrical center? There needs to be one to conform to the term.
Well, I suppose one could lift both ends of the transformer secondary and reference them to a ground point somewhere further back, but this is not true balancing. And please note my use of the word "true".
Often? Meh, maybe sometimes if a poor ground is made or to the wrong place. And more likely if there is no grounded center tapped transformer on the far end. Remember, balanced means not just hi and lo as you look at a diagram, it's end to end as well.
That may be one use, but the main reason in audio is to reduce the effects of longitudinal currents as well as the influence of external magnetic fields and line noises through the electrical center. (CMR) This is assuming there are matching ct transformers on both ends.
This I paraphrased from Howard M. Tremaine's "Audio Cyclopedia". Someone I'm sure knew more then both of us put together.
Victor
Yes true, but then where is the electrical center? There needs to be one to conform to the term.
Well, I suppose one could lift both ends of the transformer secondary and reference them to a ground point somewhere further back, but this is not true balancing. And please note my use of the word "true".
Often? Meh, maybe sometimes if a poor ground is made or to the wrong place. And more likely if there is no grounded center tapped transformer on the far end. Remember, balanced means not just hi and lo as you look at a diagram, it's end to end as well.
That may be one use, but the main reason in audio is to reduce the effects of longitudinal currents as well as the influence of external magnetic fields and line noises through the electrical center. (CMR) This is assuming there are matching ct transformers on both ends.
This I paraphrased from Howard M. Tremaine's "Audio Cyclopedia". Someone I'm sure knew more then both of us put together.
Victor
The balanced signal is not necessarily referenced to the ground and that's why the output transformer does not need to have a center tap (except in the examples given by EC8010). Examples of balanced signals (other than audio signals) include phone signals, network signals, RS485, etc.
Regards,
Milan
Regards,
Milan
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