Adding a Tape Out to a home made valve amp?

[Courtney Louise]

Hi guys, I have a home built preamp from a Chinese kit. It has 4 inputs (capacity for 6) however it doesn't have a tape output for recording the various inputs. Is this an easy thing to add as the only output already goes to the power amp.

Thanks

Courtney x

[Nickthedentist]

Let's have a circuit diagram then

Let's have a circuit diagram then

Indeed, a picture is worth a thousand words.

[Nicklyons2]

I'd be tempted to use a transformer here. RS have a range of small multi winding audio transformers which, by selecting the correct ratios should give you a reasonable impedance match and two isolated windings; banishing the possibility of hum loops and improving safety all in one go.

[majoconz]

IMHO the usual arrangement for a "Tape out" is to also have a "Playback in" socket. The "Out" comes directly after the input selector - the input selector to pre-amp wire is disconnected. The "In" socket then goes to the preamp. So if you're not using a tape deck you have to have a link between the "Out" and "In". Then with a three head tape deck you can leave it permanently connected and play through with the "Monitor" button pressed or "Playback".
If their is a magnetic cartridge amplifier in the preamp it usually comes before the selector switch so that your can record the properly equalised and amplified signal.

[Courtney Louise]

I will upload the cct tomorrow and I have been using one of the 4 inputs for tape in

[paulsherwin]

The basic problem is that valve designs normally to use high impedances and transistor gear uses low impedances. You can easily tap off an output from the volume control but it is likely to expect around a 1M input impedance. Most valve domestic tape recorders will match this using the 'radio' or 'diode' input but transistor gear will normally have an input of impedance of 100k and sometimes as low as 5k.

You can build an impedance matching circuit using valves (cathode follower) or transistors (FET designs usually have a high impedance input). Googling will give you some ideas.

[bikerhifinut]

The easy way, is as suggested by another, to take an output directly off the selector switch before any signal processing/tone controls etc. You could simply run these in parallel with the input to the volume control, or you can add a couple of "buffering" resistors in series with the output to the tape recorder. As I remember, a typical value is around 2.2k.
More sophisticated techniques would involve a changeover switch that isolated the inputs from the volume control and connected it to a separate set of input sockets for the tape recorders "line out" thus affording a monitoring facility. This is what I would do, and have done with some of my homebrew designs.
In the Morgan Jones Valve amplifiers book he suggests a more sophisticated switching system where the tape inputs are isolated until tape is selected. Thats up to you, but my feeling is you'll be fine with a simple "changeover switch".
I wouldnt worry over impedance matching. If you're using modern sources they will have a relatively low output impedance which is fine into a high impedance input and the tape recorders output should also be fine into your valve preamp, I assume you're probably using a 50k or 100k pot at the input?
I'd recommend a read of "Valve amplifiers" by Morgan Jones, its well worth the money and covers a myriad topics, all of which are useful and you don't have to go too far into the maths if you don't need to. No doubt other members here will have their own recommendations of reference tomes.
For what it costs, in my opinion, a copy of a book like "valve amplifiers" is worth its weight in gold and could save you a lot of grief.

Andy.

[joebog1]

HHMMM !!!! Paul has hit the nail on the head.
Tape out is exactly for tape. ( with valves) NOT a digital audio input!!
I have tried many "audio cards" with the same result.
ALL that I have tried ( those without an external "mixer" amplifier)
require UP TO 5 volts input to drive recording software up to my interpretation of zero dB!!!!!
Yes, almost a dead short as far as valve gear is concerned.
I also have a series of transistor amps ( that I dont use, but are too good to chuck)
that also need a low impedance source to really work well with low noise.
0dB is a "thing" that doesnt exist anymore!! Just look at PMPO amplifiers!
I have a 1000 watt PMPO speaker system for a computer that has LM386 chips!!!
In the book they are 1 ( one ) watt output.
I developed a circuit that will work, WITH valves, but needs a small external chassis and power supply.
I use a valve ( there are a few that are available) that were origionally designed for vertical amplifiers in black and white TV,s.
They are called " Dual triodes, with dissimilar sections" Consisting of a 12AX7 or 12AU7 or even a 12AT7 in the same bottle as a 12B4 or similar, small power triode.
Its very easy to design a little cap coupled plate follower amp that will do what you want it to do, without reducing the main chinese power amplifier output to maybe 1 watt and makes it sound like an old telephone. I can publish my circuit if you wish, even though its nothing "special".

Doncha just love "modern" electronics? where the standard that exists is,
"AS long as nobody else uses the same broken calculator"

With best regards
Joe

[PaulR]

Originally Posted by paulsherwin

The basic problem is that valve designs normally to use high impedances and transistor gear uses low impedances. You can easily tap off an output from the volume control but it is likely to expect around a 1M input impedance. Most valve domestic tape recorders will match this using the 'radio' or 'diode' input but transistor gear will normally have an input of impedance of 100k and sometimes as low as 5k.

You can build an impedance matching circuit using valves (cathode follower) or transistors (FET designs usually have a high impedance input). Googling will give you some ideas.

This is certainly a problem when I connect my SSTran MW transmitter to the tape output of my Sansui valve tuner/amp. The output from the receiver becomes muffled and reduced. In fact I have given up using the Sansui and use a transistor receiver instead.

[bikerhifinut]

I don't want to upset anyone here, but i feel the OP's question has been hijacked.
Courtney Louise simply asked how to wire in a "tape loop". We have to assume it's for a domestic style setup based on the info given.
So the simple answer is to wire in a set of sockets from the input selector, and if "monitoring" is required a changeover switch will suffice to route the inputs via the recorders circuit.
This is the way the majority of commercial designs do it.

Simples.

Andy.