Welborne Labs Hybrid Linestage

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Since 1987

Hybrid X
SE Tube/MOSFET Linestage Module

This SE tube/mosfet linestage module kit, designed by Erno Borbely, is now available from Welborne Labs. Its all class-A operation makes for a very sweet sounding amplifier and its low impedance mosfet output stage is good for all the bass slam you could ask for! The Hybrid X can be configured as a single-ended linestage with adjustable gain or unity gain, an I/V converter for DAC outputs, a balanced to single-ended converter, mic preamp, phono preamp, headphone amp and more. Just about any audio circuit you could conceive of using op amps can be achieved with this module.

This is a great little design. We currently have them in the output of our CD player and have recently prototyped a phonostage with the Hybrid. They both sound great!

Features:
It boasts many very useful features making it a very versatile amplifier, including: a differential tube input stage, adjustable gain and a class-A single-ended MOSFET output stage with a very low output impedance providing high current drive capabilities. MOSFET devices possess electrical characteristics that are similar to vacuum tubes and thus enhance the sonics of the driver stage. BUT, the most interesting feature of our Hybrid X design is that it uses the ECC86/6GM8 low voltage tube. The 6GM8 falls into the car radio tube category from the 1950's and 60's with specifications rated for linear operation provided with anode voltages supplied by a car battery, i.e. 12 volts. The 6GM8 can be operated at higher voltages and in fact sounds better at these elevated potentials and thus we operate the Hybrid X from a + 24V supply. You can also try the ECC88/6DJ8, 6922 and others in the Hybrid X circuit and you will be amazed at just how good these high voltage tubes sound run at low voltages.

The Hybrid X with Dale metal film resistors and Elna Cerafine Capacitors

We have packaged the Hybrid X as a mono amplifier, i.e. two boards are required for stereo operation and four boards are required for balanced stereo operation. The MINI AMP is packaged on a silkscreened 4-layer printed circuit board measuring a very petit 3"(76mm) by 3"(76(mm). This 4-layer board features a dual ground-plane design providing a very low impedance return path and superior shielding for added immunity from noise. Each unit also includes on-board voltage regulation circuitry. These regulators can be bypassed if using a power supply with a more elaborate regulation scheme. The kit includes very detailed assembly instructions and all components necessary for proper operation are individually packaged in plastic bags and marked with their reference designators.

Circuit Description:
The Hybrid X is a DC-coupled hybrid tube/MOSFET amplifier. It uses a + 24Vdc anode supply and 6.3Vdc for the heaters. The 24Vdc supply is provided by onboard voltage regulators. Its input stage is a differential amplifier using the ECC86 tube with a constant current diode supplying the cathode current.

The two anodes, which produce two out of phase signals, are converted to a single-ended signal, using a current mirror circuit composed of Q1, D2, R3 and R4. Q2, a 2SJ79 P-channel MOSFET, is used in a common-source mode as a class-A single-ended SE output stage. Q2's drain resistance has been replaced with a second constant-current source to increase its gain and linearity. This current source is made up of Q3, a 2SK216 N-channel MOSFET.

The Hybrid X Schematic Diagram

This circuit's open loop gain of 53dB is relatively high compared to most tube amplifiers and with an open loop THD measurement of 0.15%, it's also very linear. This THD is mostly all second order too! However, due to the high open loop gain, you can't use the Hybrid X without feedback. Resistors R8 and R9 apply the feedback along with capacitor C2 which applies phase compensation adjusted for best square wave response.

The feedback resistors R8 and R9 adjust the gain of the amplifier which can be set anywhere from unity-gain (gain of 1) up to 40dB. The amplifier is not unity-gain stable and therefore the R15/C3 network is provided to roll-off the open loop response of the amplifier allowing it to work with low gain settings. So if you want to use the amp with 6dB of gain or less, you need to install R15 and C3. More information on these resistor and capacitor selections are provided in the Hybrid X assembly manual.

As previously mentioned, due to the Hybrid X's design, it can be configured for many different applications.

Figure 1 depicts the amplifier connected as a non-inverting amp with a gain of 20dB. The gain is calculated as 20 times the LOG of R8 + R9 divided by R9.

Figure 1

Figure 2 shows the amplifier as a unity-gain buffer.

Figure 2

Figure 3 shows the amplifier configured as a balanced-to- single converter. Common-mode rejection is about 50dB without matching the halfs of the ECC86 and without trimming resistors. All resistors are shown here as 10Kohm however if you need a higher input impedance for interfacing with other tube circuits these values can be increased up to 100K.

Figure 3

Since the amplifier behaves like an operational amplifier, you can also use it with its negative input as a current-to- voltage I/V converter. Replace the cheap op amps in your CD player with this circuit and your system will come alive!

Figure 4

Figure 5 shows the Hybrid X used in a passive I/V converter mode with a small valued resistor on the output of the DAC to ground to create a voltage for amplifying.

Figure 5

Figure 6 is another Passive I/V converter for DACs with differential outputs.

Figure 6

Figure 7

Figure 7 depicts two Hybrid X modules configured as a phonostage with a passive RIAA network in between them. We haven't tried this yet but will be prototyping it within the next few weeks. With a closed loop THD performance of 0.005% distortion at 0.65V plus an equivalent input noise measurement of 1.2uV, this circuit should be clean enough and quiet enough to make a fine phonostage!

The assembly manual includes step-by-step instructions for assembling and testing each amp along with connection diagrams for configuring these amps for the different applications listed above. We will try to post more application info on our website soon.

Click here for an example connection diagram for a 5 input linestage using the Hybrid X and our PS5 power supply.

The fixins for a complete linestage. Just order a couple HybridX modules, a PS5 module and two PSREG modules, throw in a volume control, selector switch, rca jacks and wire... and you're there!

Ordering Instructions

Be sure to order two amps for single ended stereo operation. For balanced operation order four amps.

Basic Version includes Dale metal film resistors, Nichicon low impedance electrolytic capacitors, Wima film and foil capacitors and National Semiconductor voltage regulators.

Available Upgrades include the ELNA Cerafine electrolytic capacitors and the Linear Technology voltage regulators. BE SURE TO ORDER A QUANTITY OF TWO ON THE UPGRADES FOR STEREO OPERATION.

PRICING AND OPTIONS

HYB	Basic Hybrid X (tubes not included) - 1 Channel	$70.00

HYBCC	Cerafine Capacitor Upgrade - 1 Channel	$6.00


HYBLT	Linear Technology Voltage Regulator Upgrade - 1 Ch	$9.00

HYBA	Assemble and Test Fee - 1 Channel	$30.00

TUBE OPTIONS

ECC86/6GM8 NOS Tubes

PLEASE NOTE: At this time, these tubes are only sold to individuals that have purchased a Hybrid X kit.

We reserve the right to limit quantities on tube purchases, or in other words "don't be greedy".

I haven't had a chance to listen to all of these tubes yet so the prices don't necessarily reflect the sound quality of a particular brand. Accept for the Russian tubes all were manufactured in either Germany or Holland.

All tubes are tested, matched using a calibrated tester and warranted for 6 months.