The 6AH4 Line Stage

Hi!

Since I mostly write about the 10Y linestages (those are also requested more often) many are not aware that I also offer a smaller, single chassis linestage using indirectly heated tubes like the 6AH4.

Depending on the gain requirements this linestage can also be built with the 6J5 or 27.

This version has a slightly different design than the 6J5 linestage.

The tubes are prominently displayed at the front with all capacitors and the power transformer mounted on the top plate. The 6BY5 rectifier resides in the left corner at the back.

Here some impressions from the internals. The top plate with sockets and parts mounted and wired:

All chokes and the line output transformers reside on a separate plate, in this particular build there are also two input transformers to offer a set of XLR inputs.

The transformer volume controls are then directly mounted on the front plate by the volume control switch. Here the preamp powered up for the first time in the workshop system:

A compact solution for those who do not have the space for a 2 chassis preamp and it comes at about half the cost compared to the 10Y line.

Best regards

Thomas

The Octal Preamplifier

Hi!

since I mostly write about my separate phono and linestages, many people are not aware that I also have a full function preamp in the offering which has a phono stage integrated. The Octal Preamplifier.

The Octal Preamp has a transformer coupled lien stage with auto former volume control. In this Mk2 version the linestage uses the 6AH4 triode.

The phono section is a LC coupled circuit with passive split RC RIAA.

6SC7 is used as input tube and 6N7 in the second stage.

Signal section and power supply are in separate chassis.

In this case the preamp is built in the classic landscape chassis but could be done in portrait stye as well.

A lower cost alternative to my D3A and 10Y combo.

Best regards

Thomas

The Octal Preamplifier Mk2

Hi!

Since I just finished another build of an Octal Preamplifier in the Mk2 version, I thought this is a good opportunity to write again about my Octal Preamplifier series.

The Development of the first Octal Preamplifier started back in 2008. After I published the 6CB5A amplifier concept on a german tube forum, the question about a matching preamp came up. Since the 6CB5A amp got built in various incarnations including some no compromise versions, The question about an improved Octal Preamplifier came up as well. This led to the Mk2. The details about the design can be found here.

After all the line stages with directly heated tubes and LCR phono stages, it was refreshing to work on this design again. The circuit is unchanged from previous versions. Also the chassis design is virtually the same classic landscape layout. With just the exception that the tubes on the signal section are now on a suspended plate to reduce microphonics.

The use of such high mu and low transconductance tubes as the 6SL7 or 6SC7 as in this version, as first tube in the phono section gets unjustified criticism by some tube amplifier builders recently. The claim being that only high transconductance tubes will yield a low noise design. But in real life the few dB difference in noise performance which can be gained by using high transconductance tubes is rather minor. The 6SC7 performs nice and quiet in this design. But it doesn't have the diva like behaviour which tubes like the D3a or WE417 can show. If not taken care of they can burst into self oscillation which can cause rather harsh sound.

The 6SC7 is complemented well by the 6N7 which is used as second stage in the phono section. RIAA equalisation is done by split passive RC networks. The beefy 6AH4 is performing in the line stage. It drives a Slagle/Intactaudio AVC through a step down line output transformer.


The volume control is done by a 24 position Elma rotary switch which selects the taps on the AVC and provides 2dB steps. There are three line inputs and one phono input for MM cartridges. MCs can be connected through an external step up transformer. The MC transformer can be integrated as well but the person who is going to use this preamp already has a selection of step up transformers.

Power is provided to the signal section from a PSU in it's own chassis. As a special touch the power supply has a hybrid rectifier bridge using two 816 mercury vapour rectifiers in conjunction with two 6AX4 TV damper tubes.

The 816 is the 'small sister' of the 866A and provides a similar eerie blue glow in operation. More about this tube in the upcoming tube of the month post.

Due to the 6AX4s controlled warm up, no separate filament and high voltage switches are needed. The high voltage comes up gradually as the heaters of the 6AX4s warm up. This has the nice visual effect that the blue glow appears gradually. This is illustrated in this short video clip:

How about the sound of this preamp? As it is playing right now I am enjoying nice vocals, beautiful tone colors and a smooth sound. The 6SC7 and 6N7 combo in the phono section are performing without any harshness. No listening fatigue. The overall representation tends a very slight bit to the caramel side compared to other preamps. But in a very subtle and nice way. The line section brings all the advantages of a low impedance transformer volume control. High resolution and transparency. The low output impedance can drive any cable lengths and pretty much any power amp.

So why bother with separate phono and line stages, LCR EQ and directly heated triodes if this design performs so well? Of course such separates bring yet another level of performance. A D3a or EC8020 phono does have a more quiet back ground and improved articulation of sibilants. A direct heated line stage does give a higher level of resolution and stronger tone colors. But all that at 2.5 - 3 times the cost or more depending on some choices and twice the rack space needed. If the budget does not allow for a full fledged LCR phono and DHT line stage, the Octal preamplifier is a smarter choice, rather than trying to take short cuts and choosing a cheap implementation with LCR and DHTs.

The photos above do not show the final wooden enclosures. These are still in the making. The wooden frames on the photos are just temporary and the chassis will teals get some feet to rest on. This preamp will be used with a 6CB5A amp for which it was originally designed. The 6CB5A amp is also finished und currently undergoing testing. It will be shown in an upcoming post. Stay tuned!

Best regards

Thomas

ShellacSavor, Part 2: The Circuit

Hi!

In part 1 of the series about Shellack reproduction, I covered the basics about the different EQ curves used.  In this post I will show the circuit of a variable EQ preamplifier for Shellac playback.

In the first part the faceplate of the preamp was already shown with 12 settings each for the turnover frequency and 10kHz roll off. Turnover and 10kHz roll off shall be implemented passively and separately so that they do not interact with each other. This requires a 3 stage design. A first stage which drives the turnover EQ network and a second stage to drive the high frequency EQ. Followed by a first stage which should provide low output impedance.

Although I am in favour of LCR EQ networks, implementing a total of 24 would require a lot of space and become impractical. Therefor the EQs will be implemented with RC networks. The bass EQ will require 3 passive components. An input resistor followed by a RC segment to ground which will determine at which frequency roll off will start and stop. A fixed 100k resistor can be chosen as input feed and a selectable RC segment based on each setting.

The high frequency EQ only requires a capacitor after the input resistor. The same 100k is chosen followed by selectable capacitance to ground. The flat (0dB) position will not require a capacity so that position of the switch will be left open.

For the equalisation to remain fairly constant with ageing of the driving tubes, it is desirable that the tubes chosen have a plate resistance which is substantially lower than the input resistance of the networks. Ideally less than one tenth. The output resistance of the driving stage is in series with the EQ input resistor. This way a 10% change in plate resistance would translate to less than 1% change of the EQ input resistance. So we would want tubes with plate resistances well below 10kOhms.

Another factor which determines the tube choice is the overall gain needed. MC cartridges for Shellac records usually have a rather high output voltage compared to regular cartridges. I have chosen the Ortofon SPU MONO CG 65 which has an output voltage of 1.5mV at an internal resistance of 6 Ohms. So with a step up transformer this will deliver 10-20mV depending on the step up ratio used. The loss in the EQ networks will vary depending on the EQ setting. This can be anywhere from around 20dB to a bit over 32dB loss. The preamp shall be used with an active line stage, so a sensible raw gain (excluding step up transformer) would be in the range 60-70dB which would leave 30-50dB after the EQ, depending on the EQ setting. Adjustment of the gain could still be done with choice of MC step up transformer.

In the last tube of the month article I already presented the 6SN7, which seems perfect for the job. It has a mu of 20 and a plate resistance of 7kOhm. With resistive plate load this would give a gain of about 15 at an output impedance of around 5kohms. The two 6SN7 stages (one triode system per stage) would result in a raw gain of 225.

To bring that up to the desired total gain we need another factor of 5 or 6. A 6AH4 seems perfect for the job with it's mu of 8 and plate resistance of 1780 Ohms.

This is a sketch of the circuit:

The values of the resistors and capacitors in the EQ networks might need some tuning.

The prototype build of this circuit will be shown in part 3. Stay tuned!

Best regards

Thomas

Making of a 6AH4 Line Stage: Assembly

Hi!

As with the 6CB5A amp, which was recently presented, this second part of the 'Making of the 6AH4 Line Stage' will show the assembly process. First a photo of the finished preamp:

The circuit was already shown and discussed in the first part. This post will show how the components are arranged and wired. In the first step, the metal plate which holds all the parts is fixed in the assembly rig and the first parts get mounted: power transformer, capacitors, connectors and tube sockets:

The rig allows handling of the preamp during construction without the danger to scratch the varnished caps or damaging the plate. It can be turned at either side for easy access. The next photo shows the wiring of the power transformer to the rectifier sockets and the heater wiring to the signal tubes:

Next the inputs are wired to the input selector switch and from there to the 6AH4 tubes:

Chokes, line out transformers and transformer volume controls are mounted on a second tier:

Everything wired up, tested and complete, so the bottom lid can be put in place:

Before the bottom lid is attached the cover is mounted over the power transformer.

The preamp in operation:

 

The preamp together with the 6CB5A power amp which it is intended to drive:

Best regards

Thomas

Making of a 6AH4 line stage: circuit

Hi!

In the previous article the 6AH4 linestage was mentioned, which is meant to be used with the 6CB5A amp. As with the power amp, I will describe circuit and assembly process of the line stage in two separate articles.

The linestage shares the same circuit with the two chassis version which was already presented. Only this time power supply and preamp will be in one housing which will be in the same style as the power amp it is supposed to drive. The circuit is basically the linestage section of my Octal preamplifier Mk2. The only difference is in the heater supply, which can be AC since this is a linelevel preamp only.

The picture below shows the complete schematic with both channels and the power supply:

The TVCs are done with 24*2 db/step autoformer modules from intactaudio. Volume control and input select switches are swiss made from Elma. All capacitors are ASC MP in oil types. No electrolytics.

The only amplifying element is a 6AH4GT triode per channel. This is a low mu triode with low plate resistance. With the 3.5:1 step down this yields a very low output impedance, more than suitable to drive a TVC and long cables to the power amp.

As in the 6CB5 amp, a hybrid rectifier bridge was chosen. UF4007 diodes complement the 6AX4 TV dampers. Two diodes are in series in each leg, since substantial peak inverse voltages can occur in a choke input filter supply. The power transformer is of the same type as the one used in the power amp. Only a smaller version since the preamp has much less current draw. The power transformer has grain oriented core laminations and dual electrostratic screens between primary and secondaries. In addition there is a flux band around the outside of the winding stack.

The output is basically floating to allow differential connection to the power amp. One side is connected to ground through a 1M resistor to avoid static charge build up in the output which could cause a plop when plugged in while power amp is turned on. The heater supply of the 6AH4 is referenced to ground through 2 100 Ohm resistors.

The preamp has the same ground lift option as the power amp to allow adjustment to any grounding situation in the system.

The construction will be done on the same style metal plates as for the power amp:

Stay tuned for the second part which will show the assembly process.

Best regards

Thomas

The Modular Preamplifier, Part 10: 6AH4 Linestage

Hi!

The linestage version of the modular preamp sytsem with indirectly heated triode is finished.

It uses the 6AH4. Like all gain stages of the modular preamp it has it's own external power supply.

The circuit is exactly the same as the linestage section of the Octal preamplihier Mk2. The power supply uses the same full wave bridge with 4 6AX4 TV dampers and choke input filter. Only the heater supply is different. In the Octal preamp all heaters are supplied with DC, since it also contains a hum sensitive phono section. In a linestage it is sufficient to heat the tubes with AC. This simplifies the supply and works hum free. The heaters are just referenced to ground through two resistors across the heater line.

Here a photo which shows the inside construction of the preamp and power supply. The PSU is completed. The preamp still misses all the wiring from the inputs to the selector switch in this picture.

The linestage has 5 inputs. Four of them are regular inputs with 100kOhm input impedance for all kinds of line sources. One set of inputs goes through 1:4 step up transformers. These are reserved for low impedance sources which need some additional gain.

The two sets of output jacks are wired in parallel. Separate binding posts which connect to chassis and signal ground. A ground lift switch provides the option to either connect signal ground with chassis or have them disconnected. This allows for adjustment in case of ground loops in the system. The power supply is hooked up to the preamp trhough an umbilical with 5 pin Amphenol connectors.

The sound is the same as the linestage section of the Octal preamp Mk2. It uses the same AVC from intactaudio. Maybe a tad more refined since it has it's own dedicated power supply, which is shared with the phono section in The Octal preamp.

The next part of the modular preamp to be finished will be the linestage with the directly heated 10Y.

Best regards

Thomas