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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Good Line Preamplifier, 600 Ohm load?
Good Line Preamplifier, 600 Ohm load?
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Old 7th September 2018, 09:05 AM   #1
lineup is offline lineup  Sweden
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Default Good Line Preamplifier, 600 Ohm load?

When seeing many line peamplifiers, they can drive '600 Ohm'. But is this really necessary? I would say it is enough to be able to drive like 2k Ohm. Why should a preamplifier be able to do 600 Ohm? What is your thoughts on this? What would you say about preamplifer load? What should a good preamplifier be able to do?

(I have a project going to make a good line preamplifier.)
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Old 7th September 2018, 09:23 AM   #2
abraxalito is offline abraxalito  United Kingdom
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I don't think driving 600ohm is necessary. Its more a statement that the output stage is more than man enough for the job. Overkill tends to be desirable in audio.

Nowadays there are a few classD amps with lowish input impedances (2k-3k or so), I'd want to be sure I could drive those. If the drive voltage is just 1VRMS (a fairly typical poweramp input sensitivity) then even 600ohms needs no more than 2.5mA peak, not too hard to provide.
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Old 7th September 2018, 10:11 AM   #3
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by lineup View Post
But is this really necessary?

The lower current deviation in the any output stage - the lower distortion. Or you need to provide excessive OLG around OPS.

Quote:
Originally Posted by lineup View Post
I have a project going to make a good line preamplifier.

IMG_6511.JPG
Or just force high-GBW opamp output with simple EF having appropriate current source at the load.

Having 2k input resistanse itís easy to provide at least ~80 dB regulation with 10k linear potz in front of it:
IMG_5322.JPG
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Old 7th September 2018, 10:12 AM   #4
analog_sa is offline analog_sa  Europe
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Good Line Preamplifier, 600 Ohm load?
Quote:
Originally Posted by lineup View Post
But is this really necessary?
Most of the time not, unless you have some kind of unbuffered power amp, such as variations of some FW amps, or you have a power amp with an input 600R transformer, or excessive capacitance due to input stage or cables.

There are also some commercial amps with extremely low input impedance such as the old Linns.
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Old 7th September 2018, 10:26 AM   #5
lineup is offline lineup  Sweden
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Yes. most people would not need 600 Ohm capability. But I would say, it depends on what gear I have, what needs I have.
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Old 7th September 2018, 10:51 AM   #6
rmaudio is offline rmaudio  United Kingdom
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The '600 Ohm' standard exists for historical reasons from telecomms and dBm definition etc. Not really relevant now unless you have a particular requirement. I would generally want to be able to go into the front end of a mic preamp. That might have an input Z around 2K or so - I'd take a design value of 1K5 or so.
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Old 7th September 2018, 09:17 PM   #7
PRR is offline PRR  United States
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> want to be able to go into the front end of a mic preamp.

But never straight from a line-level?!

A 20dB pad can be devised to show over 5K to the source.

600 Ohms is still an option for some test gear.

The '5532 chip and friends will drive <600r, so a preamp-maker may as well use that in the specs.

Many modern power amps and other interfaces have input impedance as low as 10K. A monster-system might drive a dozen power amps parallel (unlikely, but it could happen). The resulting 800r load is still OK for a 600r output.

Transformers suck energy beyond their nominal impedances especially in bass. While 10K:10K would be fine for most hi-fi interfaces, 600:600 lumps are more common. Cheap "600:600" iron may be well below 600r before 50Hz (but is not a best choice for fidelity).

> a good line preamplifier

Stated JUST like that, you intend to drive "lines", miles of wire. Which does imply some grunt. However in hi-Fi we rarely face >30' 10m of "line". While this may strain a small plate, it is easy for most ample outputs. Line Inputs are mostly =>10K, and you might drive a few, or just one. Your math seems fine to me.
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Old 8th September 2018, 06:37 AM   #8
lineup is offline lineup  Sweden
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It is a difference who the preamplifer is made for. If the preamplifier is made for the market, it is good with 600 Ohm capability. If I build an amplifier for myself, I know the need and can design for this only.

It is also a question of output voltage.
Harder to drive 4 Volt across 600 Ohm than 2 Volt RMS.
The resulting is a current. Some opamps have limited current output.
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Last edited by lineup; 8th September 2018 at 06:44 AM.
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Old 8th September 2018, 12:24 PM   #9
xrk971 is offline xrk971  United States
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Good Line Preamplifier, 600 Ohm load?
600ohms is needed to match impedance of balanced lines that are needed in professional audio where mics are on stage and mixer engineer is hundreds of feet back where audience is. Those cables and gear are 600ohms to keep noise low over long runs. Nowadays they use remote wireless mics. However, there is still a need to connect long XLR patch cables between gear, and all those are designed to 600ohms.

Dedicated 600ohm balanced line driver chips are ideal for applications where you reallly need to push long cables.
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Old 8th September 2018, 01:24 PM   #10
mjurban is offline mjurban  United States
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Good Line Preamplifier, 600 Ohm load?
600 ohms is a historical artifact of the early telecom days. Back then, there was no such thing as an amplifier. The energy to drive the speaker at the receiving end came entirely from the microphone. This meant the maximum power transfer theorem came into play, and it became important to match the impedances at each end. But what impedance to use? Also back then, there was no telecom network, so early telecom traffic utilized the comparatively extensive telegraph network. In order to control transmission line reflections caused by long connections, it was necessary to match impedances to the characteristic impedance of a telegraph line. Such lines were typically 6 gauge (I think) wire strung vertically 12 inches apart on a wooden pole, and the characteristic impedance of such is right about 600 ohms. That’s where it all began, and the audio and telecom industries have been saddled with it ever since.

These days, signal transfer has become more important than power transfer, and a low impedance transmitter working into a high impedance receiver has become standard for just about everything except for runs long enough to create reflections.

That said, 600 ohms is still frequently used as an indication of drive capability, and has become a de facto measuring stick.
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