Repurposing an old HiFi amp

[amanise]

Ho Ho Ho!

Christmas has brought a sudden urge to reorganise and weed some gear.

Previously, my room monitoring was carried out with a good quality Denon HiFi amp in source direct mode fed from the stereo out L+R jacks from my PC sound card. These were sent to the phono CD inputs on the amp and all worked fine.

I'm switching to a Tascam HR4x4 audio interface which has 4 TRS line outs. I may be in for an impedance problem. The intention is to go from Tascam line outs 1&2 to Denon amp CD phono (or maybe AUX) L+R respectively.

Does anyone have a view on whether either the interface or amp will be damaged - or will the impedance differences just cause bad sound - or will it all be OK? My brain says line out is the same as speaker out from a conventional sound card - but then I have no real idea what I'm doing and thought best to check before the smoke started billowing.....

Here are the numbers from the books;

Tascam Out
Line outputs (Balanced)
US-4x4HR: LINE OUT (BALANCED) 1-2 LINE OUT (BALANCED) 3-4
Connectors:
6.3mm (1/4”) standard TRS stereo jacks (Tip: HOT, Ring: COLD, Sleeve: GND)
Maximum output level:
+20 dBu (7.746 Vrms)
Rated output level:
+4 dBu (1.228 Vrms)
Output impedance: 210 Ω

Denon Amp inputs

Input Sensitivity/Input Impedance:
CD, NETWORK, AUX, RECORDER:
100 mV / 40 kΩ/kohms (SOURCE DIRECT: Off)
100 mV / 17 kΩ/kohms (SOURCE DIRECT: On)

[Drew Stephenson]

They'll be absolutely fine.

[mellowsouls]

Line out to line in - all good. Did this myself for 20yrs til I bought some active monitors. Miss the hifi amp setup sometimes!

[James Perrett]

You should be fine but the Tascam spec doesn't really include the things that you really want to know.

If I was writing that spec then I would include the minimum input impedance that the output would like to see. A proper professional piece of gear running at a nominal level of +4dBu should be happy with a 600 ohm load - but I'll bet that the bass end probably diminishes if you try that with the Tascam. Hugh's normal rule is to make the input impedance at least 10x the output impedance.

I would also want to know exactly how the balanced output is wired so that I could choose the best way to convert it to unbalanced. Most of the time you will be fine using a T-S to phono cable but this could cause a problem with certain pieces of gear that actively drive the - output.

One other thing - an output with a nominal working level of +4dBu is likely to be a bit hot for the average hifi amp so you may want to add an in-line attenuator - or go careful with the volume control.

[amanise]

Smashing! Thanks folks! I shall look for a suitable adapter lead on the Amazon! I'd rather spend a day rooting in bins at Maplins but....

[James Perrett]

Smashing! Thanks folks! I shall look for a suitable adapter lead on the Amazon! I'd rather spend a day rooting in bins at Maplins but....

Give these guys some support. They used to work for the local Maplin shop and are now re-creating their own version.

https://www.tehonline.co.uk/

Lots of useful gadgets there too...

[amanise]

Smashing! Thanks folks! I shall look for a suitable adapter lead on the Amazon! I'd rather spend a day rooting in bins at Maplins but....

Give these guys some support. They used to work for the local Maplin shop and are now re-creating their own version.

https://www.tehonline.co.uk/

Lots of useful gadgets there too...

[Hugh Robjohns]

Impedances won't be an issue — it rarely is! — but you may have some difficulty with levels because the nominal input level for the amp is 100mV while the Tascam delivers 1.3V (+4dBu) — and that's assuming your average level is -16dBFS.

So in round numbers, the tascam is around 20dB hotter than the Denon expects.

You can make or buy in-line pads (built into phono adapters) which would help...

And it would be worth checking the Tascam manual to find what they recommend when connecting to unbalanced destinations. Some outputs prefer the cold side to be left isolated, while others prefer it to be grounded. If in doubt, leave it isolated...

[amanise]

Impedances won't be an issue — it rarely is! — but you may have some difficulty with levels because the nominal input level for the amp is 100mV while the Tascam delivers 1.3V (+4dBu) — and that's assuming your average level is -16dBFS.

So in round numbers, the tascam is around 20dB hotter than the Denon expects.

You can make or buy in-line pads (built into phono adapters) which would help...

And it would be worth checking the Tascam manual to find what they recommend when connecting to unbalanced destinations. Some outputs prefer the cold side to be left isolated, while others prefer it to be grounded. If in doubt, leave it isolated...

Thanks very much Hugh - that's put it in even sharper perspective. I see what James means by being careful with the output volume pot on the Tascam. I'm not sure there is one. I think I should check out the pads you suggest for safety's sake. I don't want to damage the Denon - consumer HiFi it might be but its a nice unit.

[Drew Stephenson]

I see what James means by being careful with the output volume pot on the Tascam. I'm not sure there is one.

The silver knob next to the phones nob.

[amanise]

I see what James means by being careful with the output volume pot on the Tascam. I'm not sure there is one.

The silver knob next to the phones nob.

Cheers - I've seen it now. Do you know if these line outs are stereo on these things? The book suggests they are but doesn't actually say so. If they are I just need to use one of them which would be handy.

[Drew Stephenson]

If it's like the US series then they're individual balanced mono outputs. So if you want a stereo output you route it to channels one and two.

[Kwackman]

Do you know if these line outs are stereo on these things? The book suggests they are but doesn't actually say so. If they are I just need to use one of them which would be handy.

There's software for the interface and you should be able to decide what each of the 4 outputs will be, so getting stereo out on O/P 1 & 2 shouldn't be a problem. Someone else will know better.
BUT, be careful where you plug O/P3 & O/P4 to.
IF you plug them to other inputs on your HIFI amp, that volume knob Drew refers to does NOT control those outputs (unless I'm reading the manual wrong), so you could get very loud audio if you choose that source on your amp! Make sure you can get to your hi-fi amp volume knob very quickly!!

[James Perrett]

Do you know if these line outs are stereo on these things? The book suggests they are but doesn't actually say so. If they are I just need to use one of them which would be handy.

To be honest, whoever is doing the technical authoring for Tascam needs to be replaced right now. Their documentation is absolutely hopeless and very sloppy at the moment.

They are calling TRS connectors stereo connectors which is very confusing because they aren't being used as stereo connectors on these line outputs. They are mono balanced outputs.

[amanise]

That's what I was expecting, for them to be mono outs and to need to use 1 and 2 assigned left and right.

Cheers all! Much appreciated!

[OneWorld]

Smashing! Thanks folks! I shall look for a suitable adapter lead on the Amazon! I'd rather spend a day rooting in bins at Maplins but....

Give these guys some support. They used to work for the local Maplin shop and are now re-creating their own version.

https://www.tehonline.co.uk/

Lots of useful gadgets there too...

Thanks for the heads-up about the Electronics Hub, they're going in my browser book marks

[ef37a]

Hi Adrian, as Hugh says you will probably find level the main problem, "impedance" almost never is.
If you are up to a bit of drilling and soldering you could put two 10K log potentiometers in an ally box and be able to attenuate and balance trim the levels into the Denon*. You might also find, paradoxically that the noise level is slightly improved!

Such a box should be sited close to the amplifier with cables no longer than about 1 to 1.5m. If you want to have a go, PM me and I will draw you something up.

*IMHO NEVER made a bad thing Denon.

Dave.

[amanise]

Hi Adrian, as Hugh says you will probably find level the main problem, "impedance" almost never is.
If you are up to a bit of drilling and soldering you could put two 10K log potentiometers in an ally box and be able to attenuate and balance trim the levels into the Denon*. You might also find, paradoxically that the noise level is slightly improved!

Such a box should be sited close to the amplifier with cables no longer than about 1 to 1.5m. If you want to have a go, PM me and I will draw you something up.

*IMHO NEVER made a bad thing Denon.

Dave.

Cheers Dave! I'll see how it goes and maybe give you a shout in the new year. I finally found the level pot for 1&2 outs in the place Drew pointed me at, so the plan is to measure the SPLs in my listening position from the current setup with my phone app - and then switch over to the Tascam and laptop and leave the amp volume pot where it always is at 9 O clock. Then when I bring up the volume up on the Tascam I'll stop when it reaches my usual monitoring level at about 80-85dBfs. I shouldn't have very much to wind on by the looks of things! After that I should be able to use both my old DAW with its soundcard still connected up via the amps CD in, and the lappy through the TASCAM hooked up where a Blu-Ray or something would go - and the monitoring levels should be the same. I'll see how much noise I get.