New tape pre-amp king!!
- Thread starter Bruce B
- Start date
steve williams
Site Founder, Site Owner, Administrator
This is actually an updated version with better specs. Hence the Mk II designation.... Myles told me Nick is proud of the measurements!
What type of EQ in there, passive or active ? and how about the output coupling, cap or transformer?
tony ma
tony ma
Sorry Tony.... that one went over my head.. The EQ is switchable NAB/IEC. Each EQ has it's own 12AT7 tube for ea. channel
Don't know about caps, transformers or coupling... I know it's fully differential if that means anything.
Ki and Myles could probably tell you more about it than I can. All I care about is what it sounds like.
From the manual:
Design :
The Doshi Tape preamplifier has incorporated design techniques and uses a combination
of solid state and tube circuits where they provide best results. The Pre-preamplifier is the
first stage of the unit and incorporates an ultra low noise preamplifier chip. This device
features near theoretical levels of noise and provides approximately 30db of gain before
the EQ sections to follow.
The eq sections use one 12AT7 tube for NAB and one for IEC eq per channel, Each dual
triode providing a gain section and a low impedance follower section to drive the passive
eq section from a extremely low impedance. The result of this topology is a powerful and
dynamic sounding eq section. The 3rd 12AT7 for each channel forms the top half of a
hybrid tube that provides gain post eq and phase splitting to drive the 12BH7/ECC99
which is used as a line driver. The result is a balanced output with extremely low output
impedance and good balance. The loading limitation for this stage is dictated by the size
of the output coupling capacitors and care should be taken to limit loading to 10k ohms or
greater, otherwise the low frequency cutoff will start to rise above 5hz and have the
potential to cause phase issues in the audible range (muddy bass, etc).
Design :
The Doshi Tape preamplifier has incorporated design techniques and uses a combination
of solid state and tube circuits where they provide best results. The Pre-preamplifier is the
first stage of the unit and incorporates an ultra low noise preamplifier chip. This device
features near theoretical levels of noise and provides approximately 30db of gain before
the EQ sections to follow.
The eq sections use one 12AT7 tube for NAB and one for IEC eq per channel, Each dual
triode providing a gain section and a low impedance follower section to drive the passive
eq section from a extremely low impedance. The result of this topology is a powerful and
dynamic sounding eq section. The 3rd 12AT7 for each channel forms the top half of a
hybrid tube that provides gain post eq and phase splitting to drive the 12BH7/ECC99
which is used as a line driver. The result is a balanced output with extremely low output
impedance and good balance. The loading limitation for this stage is dictated by the size
of the output coupling capacitors and care should be taken to limit loading to 10k ohms or
greater, otherwise the low frequency cutoff will start to rise above 5hz and have the
potential to cause phase issues in the audible range (muddy bass, etc).
Thanks Bruce, I got it, it is a passive EQ and a cap output but it is not a all tube with a IC input stage, noise should be low with IC input but it will has a feed back in there so it is not a 0 feed back
tony ma
tony ma
This is what he told me:
In order to do this I have had to utilize a new preamplifier from Analog Devices that has differential inputs and close to the theoretical limits of voltage noise. This preamplifier section feeds dual eq circuits (NAB and IEC) using one 12AT7 each. These sections have follower outputs to drive the Eq with very low output impedance thus preserving dynamics and high end response. The output of the eq section feeds a long tail pair input/follower output combination created with one 12AT7 and 12BH7 per channel. This creates a balanced output with very low output impedance (approx 130 ohms) so you can drive a mixing desk with as low as a 5k input impedance.
I have been testing this circuit for the last week and am extremely pleased with the results. It can be improved with respect to balance in the long tail pair section by using a -30v supply and increasing the tail resistor value. I have ordered the parts from digikey and will have them tomorrow.
The sound of the unit is not solidstate or tube, it is extremely linear and dynamic with a touch of liquidity in the midband. There is no feedback from input to output and it has approx 58db of gain.
In order to do this I have had to utilize a new preamplifier from Analog Devices that has differential inputs and close to the theoretical limits of voltage noise. This preamplifier section feeds dual eq circuits (NAB and IEC) using one 12AT7 each. These sections have follower outputs to drive the Eq with very low output impedance thus preserving dynamics and high end response. The output of the eq section feeds a long tail pair input/follower output combination created with one 12AT7 and 12BH7 per channel. This creates a balanced output with very low output impedance (approx 130 ohms) so you can drive a mixing desk with as low as a 5k input impedance.
I have been testing this circuit for the last week and am extremely pleased with the results. It can be improved with respect to balance in the long tail pair section by using a -30v supply and increasing the tail resistor value. I have ordered the parts from digikey and will have them tomorrow.
The sound of the unit is not solidstate or tube, it is extremely linear and dynamic with a touch of liquidity in the midband. There is no feedback from input to output and it has approx 58db of gain.
Congratualations. That looks very special!
To avoid the inaccuracy at higher frequencies with digital multimeters I use two seperate audio sections of an A80 VU connected to a simple power supply. I had them calibrated .It is easy to connect to the tape machines output,relatively compact and accurate and not in circuit when not in use. Simply switch them to "Record". They are cheap on Ebay and elsewhere.
Sorry... it's all a foreign language to me.
Maybe I can encourage Nick to clear up some questions on here.
The sound of the unit is not solidstate or tube, it is extremely linear and dynamic with a touch of liquidity in the midband. There is no feedback from input to output and it has approx 58db of gain.[/I][/QUOTE]
It should say no feed back from output back to input because in the input IC is using feed back to control the gain so it is not ( no feed back from input to output)
tony ma
It should say no feed back from output back to input because in the input IC is using feed back to control the gain so it is not ( no feed back from input to output)
tony ma
Nice find.
Even if it's not the one Nick's had implemented in his new preamp, it's got to be the new generation super preamp chip!
A quick glance at the datasheet, this little thing can have max gain of 70dB on its own with single external resistor.
Few months ago, I visited a local forum member (you know who you are...) and saw his single chip DIY phono preamp... I think the secrete is out.
The only concern (as a closet case EE who had not practiced his engineering in 20+ years...) on possible improper implementation of the chip is its wide bandwidth capabilities up to 1Mhz... If the circuit went into oscillation for some unknown reason, it could do some serious damages to one's tweeters driving the amps wild...
Ki
[/QUOTE]
It should say no feed back from output back to input because in the input IC is using feed back to control the gain so it is not ( no feed back from input to output)
tony ma[/QUOTE]
Few years back, Rich Brown and Kara Chaffee visited me with her then prototype tape head tube preamp. Trying the preamp out on Studer head with ~200mH inductance which is lower than classic Ampex heads but higher than A80VU head (~10mH), my take was the preamp just didn't have enough gain for an average audiophile system. She went back and tried many different options to add front end gain without incurring huge cost, ended up using an opamp to achieve her goal.
The reality is that other than spending tons of money as Tony had done with his custom silver winding transformers, modern opamps do make sense, IMHO. Although it isn't SOTA part compared to what's available today, Studer had implemented tape head preamp in a very small circuit board mounted right next to the repro head as a part of the headstack in A820, A812, and A810 (maybe others) using single NE5532 opamp before sending more manageable signals to EQ stage in the Repro Amp cards in the audio rack.
Ki
It should say no feed back from output back to input because in the input IC is using feed back to control the gain so it is not ( no feed back from input to output)
tony ma[/QUOTE]
Few years back, Rich Brown and Kara Chaffee visited me with her then prototype tape head tube preamp. Trying the preamp out on Studer head with ~200mH inductance which is lower than classic Ampex heads but higher than A80VU head (~10mH), my take was the preamp just didn't have enough gain for an average audiophile system. She went back and tried many different options to add front end gain without incurring huge cost, ended up using an opamp to achieve her goal.
The reality is that other than spending tons of money as Tony had done with his custom silver winding transformers
, modern opamps do make sense, IMHO. Although it isn't SOTA part compared to what's available today, Studer had implemented tape head preamp in a very small circuit board mounted right next to the repro head as a part of the headstack in A820, A812, and A810 (maybe others) using single NE5532 opamp before sending more manageable signals to EQ stage in the Repro Amp cards in the audio rack.Ki
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Hi Ki
Even a copper wound step up transformer still much better than a IC, never fail in compare at MC cart RIAA phono stage, same thing happen in tape head circuit too, different in transparency and emotion
tony ma
Even a copper wound step up transformer still much better than a IC, never fail in compare at MC cart RIAA phono stage, same thing happen in tape head circuit too, different in transparency and emotion
tony ma
Agree, Tony. However, I believe it's a matter of high cost decision for manufacturers and preferred choice for hobby people like us to use the step up transformers. Most of the old Ampex tape head preamps had step up transformers at the front end. Ki
Thanks, Ki.
Just picked-up a well-optioned HP 35665A DSA for fun.
Thought you might appreciate it
Oh! Also, met Harry -- very gregarious...thanks for putting us in touch!
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